And fix up the tests and code to adapt accordingly.
While here, update the checkout action as well.

Do both in a loop to deduplicate code.
While here, only set them if they aren't empty;
this way we don't unnecessarily pollute Vars with entries
such as `GOCOVERDIR=` when they don't do anything useful.

This can easily save hundreds of stat calls per test script,
particularly when testing Go tools.

While here, remove a few uses of the deprecated io/ioutil in cmd,
and check some missed errors in txtar-c.

This will be useful in some cmd/cue test scripts
where we want each test to create a unique remote resource
that includes the current test name as a prefix,
for the sake of more easily seeing which test created which resource.

Copied from Go commit b18b05881691861c4279a50010829150f1684fa9.