feat: allow overriding executable with environment variable

[piperswe]

This allows users to provide a path to their own tailwindcss executable, which can be handy in cases where the pre-built ones won't work (e.g. in bundix-based Nix setups).

Closes #224
Replaces/closes #226

[flavorjones]

Hi, @piperswe, thanks for the PR. I'm not sure the testing coverage here is thorough enough -- see #226 for some examples of the kinds of tests that I think are necessary when we're trying to ensure the native binaries will work correctly.

I may end up just updating #226 with the env var approach as I mentioned there ... though if you want to take a stab at increasing the test coverage here I'd be happy to give more explicit feedback.

Either way, I appreciate the nudge, and I will try to ship something this weekend.

[nanaya]

Platform check needs to be skipped when env is specified otherwise it doesn't help people running on unsupported platforms but do have nodejs which can run tailwindcss.

[flavorjones]

@piperswe Thanks again for the PR and for nudging me on this. I've updated #226 with a similar approach to what you've done here, but with a bit more test coverage for a few different edge cases. I'm going to close this in preference to that PR.