Run the unit tests on PGSO-transformed code on Linux

Summary

• The Linux arm64 and x86_64 PGSO workflows run fx's unit tests on code built through the candidate's transforms, with every function marked minsize, and fail if any test fails.
• A failed run is retried once, and a pass on the retry prints a warning naming the tests that failed first.
• The PGSO control and bitcode builds time out after 30 minutes instead of 15.
• The step adds about 30 minutes to the Linux x86_64 PGSO run and 37 to the Linux arm64 one, measured on this branch's last green runs.