cut: fix overriding of -d=

[cakebaker]

While reading the code of cut I noticed that the special case -d= can't be overridden. This PR fixes the issue.

Before the change:

$ printf "a:=b" | cargo run -q cut -f2 -d= -d:
b
$ printf "a:=b" | cargo run -q cut -f2 --delimiter== -d:
=b
$ printf "a:=b" | cut -f2 -d= -d:
=b

After the change:

$ printf "a:=b" | cargo run -q cut -f2 -d= -d:
=b

[github-actions]

GNU testsuite comparison:

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