new build logic doesn't respect `platforms:` key in CI

[jhheider]

pkg.yml properly uses get-matrix.ts, but ci.yml does not, resulting in spurious build failures:

https://github.com/pkgxdev/pantry/actions/runs/7558241788

[mxcl]

There is code to allow for this, specifically bk build exits with 3 if it’s a noop due to wrong platform. The action then reads this and should set a specific output that then causes the rest of the steps to fail.

This used to work, so i guess I better figure out what changed.

[mxcl]

Well actually it does work, here is it working for the swift PR: https://github.com/pkgxdev/pantry/actions/runs/7521507863/job/20472428365#step:6:64

[jhheider]

with linux-pam, it has a dep that can't be installed, since the dep is also platform locked; so it doesn't get that far. thorny.

[mxcl]

K I get it, it's because it tries to compute the hydrated dependency graph and some of the deps have no mac versions so it fails before it realizes that it should have already aborted.

[mxcl]

so we could compute the matrix like we do for deployment jobs. Maybe we should just do that. This should work all the same tho.

[jhheider]

yes, it just needs to check platforms earlier, most likely. before it bothers with deps or anything else.

[jhheider]

doing that earliest in brewkit will be least surprising to devs.