os: make os.cache_dir() and os.vtmp_dir() more robust to parallel test executions, extract common code

[spytheman]

Intended to fix failures similar to this one:
https://github.com/vlang/v/actions/runs/7487373820/job/20387668781

 FAIL  [ 153/1701]  2033.642 ms vlib/flag/usage_example_test.v
V panic: mkdir failed for "C:\Users\runneradmin\.cache", because CreateDirectory 
returned: Cannot create a file when that file already exists.

[JalonSolov]

Seems like an odd error message, to see "cannot create a file" when you called CreateDirectory. Is that coming from Windows?

[spytheman]

Seems like an odd error message, to see "cannot create a file" when you called CreateDirectory. Is that coming from Windows?

Yes, it is weird, and yes, it is from Windows (a msvc job).