`mktemp`: expose functionality for nushell

[tskinn]

As per #5088 (comment) I'm opening this ticket to discuss details about how expose mktemp functionality for use in nushell.

[tskinn]

Should a new pub fn be created that returns a result of String as I think the current functions just print to stdout. What other considerations are there?

[tertsdiepraam]

Yes, I think that's a good idea! I think it should be roughly done as follows:

• mktemp: returns a path and calls exec and dry_exec
• exec and dry_exec should also return the path.
• uumain calls mktemp and prints the result.

Otherwise I think this one should be fairly simple. Thanks for picking this up!