[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 b1fa143dd8.

nvm.sh

@@ -2247,7 +2247,7 @@ nvm_get_mirror() {
   esac
 
 
-  if ! nvm_echo "${NVM_MIRROR}" | command awk '{ $0 ~ "^https?://[a-zA-Z0-9./_-]+$" }'; then
+  if ! nvm_echo "${NVM_MIRROR}" | command awk '{ if ($0 !~ /^https?:\/\/[a-zA-Z0-9.\/_-]+$/) exit 1 }'; then
       nvm_err '$NVM_NODEJS_ORG_MIRROR and $NVM_IOJS_ORG_MIRROR may only contain a URL'
       return 2
   fi