internal/mod: TestWorkParallel flake on windows with Go 1.20

[mvdan]

https://github.com/cue-lang/cue/actions/runs/6814173381/job/18530431005

$ go test ./...
[...]
--- FAIL: TestWorkParallel (1.02s)
    work_test.go:53: par.Work.Do does not seem to be parallel
FAIL
FAIL	cuelang.org/go/internal/mod/mvs/internal/par	1.647s

This just made CI on master fail; it seems to be a sort of flake, since on windows with Go 1.21 with the same commit it passed, as well as both windows workflows on other nearby commits.

If this keeps happening, we should skip or disable the test. Interestingly enough, the test is exactly the same in upstream Go, but they don't seem to be running into flakes.

cc @rogpeppe