t0301: another fix for Windows compatibility

Just like 0fdcfa2 (t0301: fixes for windows compatibility,
2021-09-14) explained, we should not call `mkdir -m<mode>` in the test
suite because that would fail on Windows (because Windows has a much
more powerful permission system that cannot be mapped into the simpler
user/group/other read/write/execute model).

There was one forgotten instance of this which was hidden by a `SYMLINK`
prerequisite. Currently, this prevents this test case from being
executed on Windows, but with the upcoming support for symbolic links,
it would become a problem.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

t/t0301-credential-cache.sh

@@ -123,7 +123,8 @@ test_expect_success SYMLINKS 'use user socket if user directory is a symlink to
 		rmdir \"\$HOME/dir/\" &&
 		rm \"\$HOME/.git-credential-cache\"
 	" &&
-	mkdir -p -m 700 "$HOME/dir/" &&
+	mkdir -p "$HOME/dir/" &&
+	chmod 700 "$HOME/dir/" &&
 	ln -s "$HOME/dir" "$HOME/.git-credential-cache" &&
 	check approve cache <<-\EOF &&
 	protocol=https