...for improved cross-platform support.
Following up on the work in c90ac7b. There was one more test that had mktemp in
the littlecheck "shebang" and this also removes a now-unnecessary `env` prefix.
All usages of `mktemp` must go through the (fish-only) `mktemp` test function
that abstracts over the differences across multiple platforms/flavors.
Tests can be easily run individually via `ninja -C build test_xxx` and there
isn't a good reason to randomly manually override $HOME and $XDG_CONFIG_HOME for
a test here and a test there.
If it's absolutely necessary, littlecheck.py should be extended to support a
`%temp` variable initialized to a temporary directory and that can be used
instead of calling out to the platform-provided `mktemp` via a subshell.
This was requested by a team member who would like for some tests to
remain invokable (in thier own $HOME) directly via littlecheck without
relying on the test driver to prep the environment.
A comment explaining the rationale is also added so this doesn't get
passed down as folklore "you need to include this for tests to run" even
though no one understands why.
