64:crabble-start proposal is triggering SDK rebuild

[turadg]

Evaluating the Crabble proposal is triggering SDK rebuild:

2024-02-05T19:02:40.6261224Z #1960 [test-upgrade-13 eval-crabble-start 6/6] RUN ./run_eval.sh 64:crabble-start
2024-02-05T19:02:40.6280827Z #1960 136.9 At least package.json is newer than node_modules
2024-02-05T19:02:40.6281232Z #1960 136.9 yarn install v1.22.19
…
2024-02-05T19:02:40.6288774Z #1960 136.9 Yarn packages need to be built
2024-02-05T19:02:40.6289111Z #1960 136.9 yarn run v1.22.19
2024-02-05T19:02:40.6289400Z #1960 136.9 $ yarn workspaces run build

That triggers rebuilding even Xsnap. Totally not necessary and flakes sometimes in the AMD build.

These were attempts to prevent that:
feat: absolute path to agd binary
chore: use unreleased synthetic-chain package

But they didn't work, as demonstrated in https://github.com/Agoric/agoric-3-proposals/actions/runs/7789170001/job/21240169503?pr=88

This might be due to something in #88 but needs further investigation

[turadg]

Fixed by a4020ce