env: remove the --file extension

[krosci]

uutils env accepted a -f/--file extension that loads .env-style files, which GNU env does not have. GNU 9.12 added --env0-from as the supported way to read the environment from a file, added here in #14626, so the divergent extension and its INI loader go away along with the rust-ini dependency.

Fixes #14610.

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/pr/bounded-memory is now passing!
Skip an intermittent issue tests/cut/bounded-memory (was skipped on 'main', now failing)