dircolors: replace if/else chain with match

[cakebaker]

No description provided.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tty-eof (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/timeout/timeout-group (passes in this run but fails in the 'main' branch)
Skip an intermittent issue tests/tail/tail-n0f (was skipped on 'main', now failing)

[sylvestre]

clippy improvement to catch this: rust-lang/rust-clippy#17232