Dispatch MM unit test workflows based on modular-magician downstream SHAs instead of branches

[melinath]

What kind of contribution is this issue about?

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Details

We currently dispatch workflows like https://github.com/GoogleCloudPlatform/magic-modules/blob/main/.github/workflows/test-tgc.yml based on a branch, which can lead to issues with TGC specifically where it can end up installing an older version of TPG (and then failing to compile) - for example, https://github.com/GoogleCloudPlatform/magic-modules/actions/runs/8759519595/job/24042712410.

This could also hypothetically lead to race conditions if multiple commits are pushed close to each other, resulting in unit tests running for a newer SHA than intended.

We should pass specific SHAs to GHA workflows - and for TGC we should pass a SHA for TPGB in addition to one for TGC.

Ideally we would pass the downstream SHAs from the cloud build step that commits them to the steps that consume them, but I'm not sure offhand whether cloud build supports that.

References

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