See ee6f7667 / #3067

In theory, `npx nvmrc` can now be used to validate an `.nvmrc` file that `nvm` will support. Allowances have been made for future extensibility, and aliases may no longer contain a `#`.

Fixes #3336. Closes #2288.

Co-authored-by: Jordan Harband <ljharb@gmail.com>
Co-authored-by: Yash Singh <saiansh2525@gmail.com>

`nvm unload` will unset NVM_DIR. So gotta save NVM_DIR first.

`u` -> `you`

See #3239; Fixes #3247; Closes #3246

Fixes #3239

See https://github.com/Vampire/setup-wsl/issues/50 / https://github.com/actions/runner-images/discussions/8791

This should only annoy those testing their PRs; real users should never set this env var

This should be improved in the future, but is fine for now

 - disable failing Alpine tests
 - disable Ubuntu-18.04 tests on node 18+
 - add Ubuntu-20.04 tests

avoids `expr: warning: ^lts/-[1-9][0-9]*: using ^ as the first character of a basic regular expression is not portable; it is ignored`