Fix incorrect check for "$NVM_DIR/nvm.sh"

[emparq]

Should be using -f if checking for a file.

[ljharb]

This appears to be failing every "without curl" installation test.

[emparq]

That's odd. Though I'm pretty sure those failures have nothing to do with this 1-liner, seeing as how it's an isolated if/else where echo is the only thing performed.

-  if [ -d "$NVM_DIR/nvm.sh" ]; then
+  if [ -f "$NVM_DIR/nvm.sh" ]; then
     echo "=> nvm is already installed in $NVM_DIR, trying to update the script"
   else
     echo "=> Downloading nvm as script to '$NVM_DIR'"
   fi

[ljharb]

That is a very good point. The failures are definitely unrelated to your change. Please rebase on top of latest master, and I'll merge it in.

[emparq]

Oh, hey- my bad. Sorry I didn't see the rest of your comment requesting a rebase. Thanks for taking care of this!