Avoid non reproducible cache generation at codcov

[oech3]

1. Nightly dep is not friendly for everyone (Code coverage instructions require nightly toolchain due to -Z flags #9889)
2. Avoid discarding sccache for codcov daily
   3. Avoid changing codcov result daily

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/head/head-write-error. tests/head/head-write-error is passing on 'main'. Maybe you have to rebase?
GNU test failed: tests/tail/retry. tests/tail/retry is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/shuf/shuf-reservoir (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/sort/sort-stale-thread-mem (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/symlink (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/rm/many-dir-entries-vs-OOM is now being skipped but was previously passing.
Congrats! The gnu test tests/expand/bounded-memory is now passing!

[ChrisDryden]

Is it because the flag that we needed to get the coverage data is now in stable?

[ChrisDryden]

Whatever the flag was, the data is still being passed through and working supposedly: https://app.codecov.io/gh/uutils/coreutils/pull/10926

[oech3]

flags are still nightly. RUSTC_BOOTSTRAP is a hack to enable them.
We should not discard cache (or disable sccache) anyway.