env: support --env0-from

[wtcpython]

Closes #14602.

Support --env0-from=FILE to read environment variables from a NUL-delimited file, matching GNU env 9.12.

[Copilot]

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[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Skip an intermittent issue tests/tail/pid-pipe (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/resolution (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/cut/bounded-memory is now being skipped but was previously passing.
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.

[oech3]

#14626 was submitted at 1st