mktemp: replace workaround code with features of clap 3

[jfinkels]

This code has a comment describing a workaround for a missing feature in clap that was not available until clap 3:

coreutils/src/uu/mktemp/src/mktemp.rs

Lines 89 to 98 in bf3a86f

	// Special case to workaround a limitation of clap when doing
	// mktemp --tmpdir apt-key-gpghome.XXX
	// The behavior should be
	// mktemp --tmpdir $TMPDIR apt-key-gpghome.XX
	// As --tmpdir is empty
	//
	// Fixed in clap 3
	// See https://github.com/clap-rs/clap/pull/1587
	let tmp = env::temp_dir();
	(tmpdir, tmp)

Perhaps some code here can be simplified now that clap 3 is available?

[djhworld]

I'm not sure if the linked issue in clap 3 fixes the problem.

The issue stems from the fact that --tmpdir will never be empty if a template is passed (e.g. mktemp --tmpdir apt-key-gpghome.XXX)

clap will interpret the value of --tmpdir to be apt-key-gpghome.XXX rather than "" hence why this workaround is in place.

Adding default_missing_value doesn't fix it because --tmpdir is always going to be populated when a 'template' is passed.