Fix flaky tests that use std::env::set_var

[a-kenji]

Fix flaky tests that use std::env::set_var.

std::env::set_var is not safe to be used in parallel.

Add #[ignore] statements to these tests.
Add an extra test step to ci.sh, as I don't think there is a better way mark certain tests as single threaded only.

Reproducer:

for _ in {0..100}; do
  cargo t current_dir || exit 1
done

Will fail very fast.

Doesn't fail:

for _ in {0..100}; do
  cargo t current_dir_prefers_pwd_env_var || exit 1
  cargo t current_dir_uses_dereferenced_path_when_pwd_env_var_not_set || exit 1
done

Reference: rust-lang/rust#27970

[srounce]

I'm wondering how either the tests or the implementation could be improved to make it more amenable to being tested normally? The test parallelism problem was a bit of oversight on my part, though it'd be nicer to not resort to workarounds for a less than optimal initial solution and instead have a better solution.

[zimbatm]

Do you have a better idea? Otherwise, I would propose to merge this PR until we find a better solution.

[Mic92]

@bors merge