Only run atime tests on non-fork branches

[MichaelChirico]

Observed in #4883 -- PRs originating from a fork will fail:

https://github.com/Rdatatable/data.table/actions/runs/10734222605/job/29768987281?pr=4883

Run git switch "${GITHUB_BASE_REF}"
# Previous HEAD position was 34257529 Merge 578abecb6b580da7fed2c80b4343c4d1a6b8717e into b56682218696ea6970e189e5f55cf0102a516121
# Switched to a new branch 'master'
# branch 'master' set up to track 'origin/master'.
# fatal: invalid reference: named_lapply
# Error: Process completed with exit code 128.

The alternative is to try and get this running on fork-originated PRs, I'm not sure it's needed.

[tdhock]

excellent idea, thanks, I didn't know you could do that.

[Anirban166]

Seems reasonable, but I think we should add a note directly in .github/workflows/performance-tests.yml (or .ci/atime/tests.R if appropriate enough) to remove the condition for people who intend to test changes in their fork (GHA will get skipped otherwise).

[tdhock]

I dont think it is possible (permissions) for this to run on forks