panic.rs: use var_os

[xtqqczze]

No description provided.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tty/tty-eof (passes in this run but fails in the 'main' branch)
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)

[xtqqczze]

CICD / Code Coverage (ubuntu-latest, unix) (pull_request) failure tracked by #11736.

[cakebaker]

Thanks!