Fix intermittent flow failure on CI

[mourner]

Flow started failing very often on Circle CI recently — like 50% of the time. It would be nice to figure out what's going on because having to constantly rerun workflows is annoying.

Example: https://circleci.com/gh/mapbox/mapbox-gl-js/39716

Failure log:

$ build/run-node build/generate-flow-typed-style-spec && flow .
Launching Flow server for /root/mapbox-gl-js
Spawned flow server (pid=86)
Logs will go to /tmp/flow/zSrootzSmapbox-gl-js.log
Monitor logs will go to /tmp/flow/zSrootzSmapbox-gl-js.monitor_log
Please wait. Server is initializing (parsed files 0): /Please wait. Server is initializing (parsed files 4000): -Please wait. Server is initializing (parsed files 10000): \Please wait. Server is initializing (parsed files 14000): |Please wait. Server is initializing (parsed files 18000): /Please wait. Server is initializing (parsed files 18000): -Please wait. Server is initializing (parsed files 22000): \Please wait. Server is initializing (parsed files 28000): |Please wait. Server is initializing (parsed files 32000): /Please wait. Server is initializing (parsed files 40000): -The flow server is not responding (3 retries remaining): /Launching Flow server for /root/mapbox-gl-js
Spawned flow server (pid=185)
Logs will go to /tmp/flow/zSrootzSmapbox-gl-js.log
Monitor logs will go to /tmp/flow/zSrootzSmapbox-gl-js.monitor_log
Please wait. Server is initializing (parsed files 0): /Please wait. Server is initializing (parsed files 3000): -Please wait. Server is initializing (parsed files 10000): \Please wait. Server is initializing (parsed files 15000): |Please wait. Server is initializing (parsed files 22000): /Please wait. Server is initializing (parsed files 30000): \Please wait. Server is initializing (parsed files 37000): |Please wait. Server is initializing (parsed files 41901): /The flow server is not responding (3 retries remaining): \Launching Flow server for /root/mapbox-gl-js
Spawned flow server (pid=287)
Logs will go to /tmp/flow/zSrootzSmapbox-gl-js.log
Monitor logs will go to /tmp/flow/zSrootzSmapbox-gl-js.monitor_log
Please wait. Server is initializing (parsed files 0): \Please wait. Server is initializing (parsed files 8000): |Please wait. Server is initializing (parsed files 14000): /Please wait. Server is initializing (parsed files 20000): -Please wait. Server is initializing (parsed files 25000): \Please wait. Server is initializing (parsed files 31000): |Please wait. Server is initializing (parsed files 35000): /Please wait. Server is initializing (parsed files 40000): -The flow server is not responding (3 retries remaining): |Launching Flow server for /root/mapbox-gl-js
Spawned flow server (pid=388)
Logs will go to /tmp/flow/zSrootzSmapbox-gl-js.log
Monitor logs will go to /tmp/flow/zSrootzSmapbox-gl-js.monitor_log
Please wait. Server is starting up: -Please wait. Server is initializing (parsed files 0): |Please wait. Server is initializing (parsed files 4000): /Please wait. Server is initializing (parsed files 10000): -Please wait. Server is initializing (parsed files 13000): \Please wait. Server is initializing (parsed files 18000): |Please wait. Server is initializing (parsed files 22000): /Please wait. Server is initializing (parsed files 27000): -Please wait. Server is initializing (parsed files 33000): |Please wait. Server is initializing (parsed files 37000): /Please wait. Server is initializing (parsed files 41901): -Lost connection to the flow server (0 retries remaining): \Out of retries, exiting!
error Command failed with exit code 7.
info Visit https://yarnpkg.com/en/docs/cli/run for documentation about this command.
Exited with code 1

Flow log (managed to get it through an SSH-enabled build that failed):

Mar 21 09:01:29.974 [info] argv=/root/mapbox-gl-js/node_modules/flow-bin/flow-linux64-v0.91.0/flow start --flowconfig-name .flowconfig --from parent cmdline: node /root/mapbox-gl-js/node_modules/.bin/flow . --temp-dir /tmp/flow /root/mapbox-gl-js
Mar 21 09:01:29.974 [info] Creating a new Flow server
Mar 21 09:01:34.979 [error] Closing connection 'server #1' due to uncaught exception in read loop: End_of_file
Mar 21 09:01:34.979 [error] backtrace:
Mar 21 09:01:34.979 [error] Raised at file "map.ml", line 130, characters 10-25
Mar 21 09:01:34.979 [error] Called from file "src/unix/lwt_engine.ml", line 318, characters 8-36
Mar 21 09:01:34.980 [error] 
Mar 21 09:01:35.069 [error] Flow server (pid 391) exited with code Unknown_error (110)
Mar 21 09:01:35.069 [info] Monitor is exiting (Dying along with server)
Mar 21 09:01:35.069 [info] Broadcasting to threads and waiting 1 second for them to exit

[samanpwbb]

@mourner it looks like this is still around after #8061 (https://circleci.com/gh/mapbox/mapbox-gl-js/39835?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link, https://circleci.com/gh/mapbox/mapbox-gl-js/41015?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link)