stdbuf: honor custom cargo profiles for libstdbuf

[favilances]

Summary

Fixes #12434.

stdbuf manually builds libstdbuf from build.rs so the shared library is available for include_bytes!. The nested cargo invocation only distinguished debug/release builds, so cargo build --profile=release-small -p uu_stdbuf still built libstdbuf with the release profile.

This derives the active profile from OUT_DIR and forwards custom profiles to the nested cargo build with --profile.

Tests

• cargo fmt --check -- src/uu/stdbuf/build.rs
• cargo check -p uu_stdbuf --profile release-small
• cargo test -p uu_stdbuf --profile release-small

[oech3]

Sorry. But this PR is duplicated.

[oech3]

Invalid unicode is not supported.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/dd/no-allocate is now being skipped but was previously passing.
Congrats! The gnu test tests/expand/bounded-memory is now passing!

[xtqqczze]

Duplicate of #12462