Move CI jobs to GHA

[afrittoli]

Feature request

Prow will stay around for tide and other helpful services for now, but it will not be available anymore soon to run CI jobs. Please migrate them to GHA

[jkhelil]

@afrittoli isnt this done here #1386 ?

[afrittoli]

For the e2e tests it is, but there are still a few jobs on prow (build, unit tests etc)

[AlanGreene]

I've opened #1446 to move the build and unit tests jobs, although the tests are currently failing on main.

[AlanGreene]

build and unit tests jobs are now passing and merged:

• build: https://github.com/tektoncd/chains/actions/runs/19326880726/job/55280083413
• unit tests: https://github.com/tektoncd/chains/actions/runs/19326880726/job/55280083444