[Fix] `nvm_get_mirror`: fix awk URL validation to actually reject invalid URLs

The awk expression `$0 ~ "regex"` as a bare statement in the action block evaluates the match but doesn't affect the exit code.
awk always prints the line and exits 0, making the validation a no-op.

Bug introduced in https://github.com/nvm-sh/nvm/commit/b1fa143dd8cbebd9847972c08fb383646ca00642.