Add spack env activate --temp

[haampie]

Creates an environment in a temporary directory and activates it, which
is useful for a quick ephemeral environment:

$ spack env activate -p --temp
[spack-1a203lyg] $ spack add zlib
==> Adding zlib to environment /tmp/spack-1a203lyg
==> Updating view at /tmp/spack-1a203lyg/.spack-env/view

[haampie]

This pr requires #25729 (which is merged)

[haampie]

Review would be appreciated

[tldahlgren]

Any chance of getting at least a unit test for create_temp_env_directory?

[tldahlgren]

Force re-running checks due to odd 'failure' in required shell test. (Possibly patch coverage related?)

[haampie]

Hi Tamara, I introduced that create_temp_env_directory as a small / separate function so that I could avoid running tempfile.mkdtemp in the unit tests 😅

The code should be covered with the shell test... not sure why it's failing though.

[haampie]

The shell test failure was a legitimate one.

I couldn't convince argparse to accept either --temp OR a required env name / dir, so all options were optional, and the input validation should then be done by hand.

With the last commit:

$ spack env activate
==> Error: spack env activate requires an environment name, directory, or --temp

Edit: thx @tldahlgren, there actually was a missing shell test for spack env activate --temp... hopefully coverage goes up now.