Nightly dist_linux is broken by shards

[straight-shoota]

Building shards on the latest nightly run of dist_linux and dist_linux32 breaks:

https://app.circleci.com/pipelines/github/crystal-lang/crystal/6114/workflows/f56b1316-4d01-4c00-9265-a3d31d3f14cd/jobs/61739
https://app.circleci.com/pipelines/github/crystal-lang/crystal/6114/workflows/f56b1316-4d01-4c00-9265-a3d31d3f14cd/jobs/61737

This is still with the old shards release 0.14.1. I can't see any connection to the new release 0.15.0, although it looks somewhat suspicious that it appears in the CI run after 0.15.0 was released.

The error seems to be caused by the Makefile recipe for shard.lock, which fails because shards binary is not available (SHARDS=false). This recipe should not be used because shard.lock exists and it should not be older than shard.yml. However, I think that's not necessarily guaranteed. So the cause could be a coincidental mismatch between shard.yml and shard.lock timestamps while checking out the repository. But it appears in both jobs simultaneously. That's also suspicious.

The fix should be easy (avoid running $(SHARDS) update if $SHARDS = false). But it's strange why this error appears suddenly.

[straight-shoota]

This also reproduces when I run the distributions scripts locally.