Add explicit check for $TMPDIR before calling mktemp in merge-squashfs

[schrockwell]

This is a follow-up to #778 and a proposed fix for #781.

Here's what I know:

• mktemp on macOS 13 does not support the -p option (as reported in #781)
• The -t option is problematic
  • The option behaves wildly differently between macOS and Linux
  • Ubuntu 18 manpages claim the -t option is deprecated
  • The original fallback invocation of mktemp -d -t 'merge-squashfs-tmp' doesn't even work on Linux; the template string requires "X" characters as placeholders
• mktemp does not consistently honor $TMPDIR
  • Linux mktemp honors $TMPDIR by default
  • macOS mktemp does NOT honor $TMPDIR by default

So, my proposed solution avoids -t and -p and is about as simple and explicit as possible, and it has been tested as working equally well on macOS 13, 14, and Ubuntu 18, and with or without $TMPDIR being set.

[fhunleth]

@schrockwell Thank you for this alternative implementation! It makes a lot of sense to me and you've tested it well. I'm going to test it in a few more places and hopefully we can get this in.

[fhunleth]

Thanks again for fixing this!