openbsd.yml: use system LibreSSL to save build time

[oech3]

Closes #14640

[xtqqczze]

Should we exclude vendored openssl in Cargo for OpenBSD?

[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/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)

[oech3]

Vendored openssl prbably needs C-toolchain (#14449). However, I still think testing system libressl only on the CI (to save time) and setting vendored by default it still reasonable until someone submit official uutils-coreutils package to OpenBSD.

(Our CI for MSYS2(Cygwin) tests system openssl only since the is no actual consumer for vendored openssl).

[xtqqczze]

We might need a change to .cargo/config.toml:

[target.x86_64-unknown-openbsd]
linker = "x86_64-unknown-openbsd-gcc"

[oech3]

I don't want to override user's choice (gcc, clang, etc...).

[xtqqczze]

setting vendored by default it still reasonable until someone submit official uutils-coreutils package to OpenBSD.

I think we should fallback to pure Rust implementation for now, to make clippy happy.

[oech3]

I disagree with such hack.

[xtqqczze]

The OpenBSD CI failure was unrelated, I think this change makes sense though.

[cakebaker]

Thanks!