More efficient verification of multi-subject generic provenance

[laurentsimon]

trafficstars

I added https://github.com/slsa-framework/example-package/blob/main/.github/workflows/e2e.generic.schedule.main.multi-subjects.slsa3.yml

temporarily to test verification of multiple subjects. It currently calls the verification scripts 3 times, meaning that it compiles the verifier 3 times.

We need to update the scripts to verify the 3 artifacts with a single call.

/cc @ianlewis

[laurentsimon]

2 options to achieve this:

1. Rename BINARY env variables to BINARIES, which can take a list of binaries separated by a ,. The scripts can split the string and extract the list of binaries
2. Cache the verifier binary, and only compile / download if it's not already on disk

I'm in favor of Option 2 because it keeps the scripts simpler.

Wdut?