bat cache --clear should clear the --target dir if specified

[aaronkollasch]

While bat cache --clear accepts a --target dir argument, it always clears the default cache dir regardless of --target.

Instead, bat cache --clear should clear the --target dir if one is provided.

Fixing this would allow the bat cache --clear function to be tested in an temporary directory. The relevant test is already written and can be enabled by removing the ignore flag on the cache_clear function:

bat/tests/integration_tests.rs

Lines 898 to 902 in 78a67ac

	#[test]
	#[ignore]
	fn cache_clear() {
	let src_dir = "cache_source";
	let tmp_dir = tempdir().expect("can create temporary directory");

Originally posted in #2381 (comment)