Downstream check should include changes from the generator

[blakeli0]

We currently have downstream check that validates any changes in Gax or shared dependencies would not break downstream(java-bigtable etc.) libraries. However, it does not include changes from the generator, which caused the downstream libraries to fail. It happened recently for bigtable in the generator update PR.
It was not possible to do so in the past due to BazelBot and Owlbot are not in this repo. Now that we have hermetic build, we can test downstream libraries with changes from the generator as well.

[blakeli0]

@lqiu96 Feel free to add more info.
cc: @JoeWang1127 @diegomarquezp

[diegomarquezp]

@blakeli0 does this involve downstream checks for changes in the hermetic_build/ folder?

[JoeWang1127]

@blakeli0 does this involve downstream checks for changes in the hermetic_build/ folder?

I believe so as I consider hermetic build is part of the generator.

[blakeli0]

@blakeli0 does this involve downstream checks for changes in the hermetic_build/ folder?

I believe so as I consider hermetic build is part of the generator.

Agreed. If we are going to add downstream check for the GAPIC generator, we probably only need to re-build the image to test the changes in the hermetic_build/ folder.

[diegomarquezp]

Ok, thanks @blakeli0 @JoeWang1127.

I'm then giving this a +1 to avoid toil from manual testing such as what I did with HW repos in #3723 (it may be a rather special case but still checks out IMO)