Suppress "not on PATH" warning when --prefix is given

[nchepanov]

Closes: #9821

Similar to how it works with --target, avoid printing the
warning since it's clear from the context that the final
destionation of the executables is unlikely to be in the PATH.

[uranusjr]

Looks to me, tests need to be fixed.

[nchepanov]

tests need to be fixed.

I'm not sure how, please advice:

The test_installing_scripts_outside_path_prints_warning does exactly this, it uses --prefix and expects --no-warn-script-location in stderr. Since this is the behavior we are changing, it would make sense to somehow trigger this warning by other means. But there are no other command line flags I see other than --target and --prefix that would get the warning printed. Thoughts?

The failing test is followed by succeeding but now redundant test:

• test_installing_scripts_outside_path_can_suppress_warning - the suppression no longer needed in case of --prefix Should it be removed? Changed?

@pradyunsg looks like you were the one to write these tests 4 years ago, thoughts?

[pradyunsg]

changed?

I think that test should be changed -- it's intended to check that when the scripts go somewhere that's not on PATH, the warning is printed. --prefix was being used to change where the scripts go, the test should probably be updated to instead change what PATH is (so that where the scripts go is not on PATH).

[pradyunsg]

Linters aren't happy FYI.