Retry TestBuildPowerShellCatchesExceptions get_sources failure

What does this MR do?

Sets GET_SOURCES_ATTEMPTS=3 on TestBuildPowerShellCatchesExceptions's build, giving its get_sources stage a retry budget.

Why was this MR needed?

This test runs get_sources multiple times against a real remote per subtest with no retry budget. Observed failing intermittently on both Windows (https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/15521550298) and Linux (https://gitlab.com/gitlab-org/gitlab-runner/-/jobs/15520663205) CI, with a different subtest failing each time - not tied to a specific code path. Both show the same signature: the mocked custom executor's get_sources step exits 1 with zero captured output, unlike the plain git/HTTP errors seen in #39629 (closed). The exact cause wasn't pinned down (I didn't want to guess and misdiagnose it the way an earlier fix this session initially did for #39627 (closed)), but a retry budget covers it regardless of cause since it surfaces at the get_sources stage either way.

This test was also part of the original 27 tests skipped in !6575 (closed) (#39338 (closed)), so it has a long history of intermittent failure.

Used 3 rather than 2 (unlike the earlier fixes in this series) to match defaultPullMaxAttempts in the image-pull retry manager - the existing convention elsewhere in this codebase for retrying a transient external-dependency failure. A git fetch of this small fixture repo is cheap per attempt, so the extra attempt costs little.

No production default changed.

What's the best way to test this MR?

go build -tags integration ./executors/custom/... and golangci-lint both pass with no new issues.

What are the relevant issue numbers?

Closes #39630 (closed)