utils/ruby.sh: simplify and fix Ruby-related logic

General:
- Rename `unusable_ruby` to `need_vendored_ruby` (due to code changes).

**`setup-ruby-path`** function:
- Move `-z "$HOMEBREW_RUBY_PATH"` and `-n "$HOMEBREW_FORCE_VENDOR_RUBY"` checks to `need_vendored_ruby`.

**`need_vendored_ruby`** function:
- Check `HOMEBREW_FORCE_VENDOR_RUBY` first so that it's respected even
  on Macs that are "new enough" (`HOMEBREW_MACOS_SYSTEM_RUBY_NEW_ENOUGH` is set).
- Don't check `-n $HOMEBREW_RUBY_PATH` because it is implicitly tested
  by `test_ruby`.
- Remove (now-) redundant `-z $HOMEBREW_FORCE_VENDOR_RUBY` check as it is
  tested earlier in the conditional.

Library/Homebrew/utils/ruby.sh

@@ -28,12 +28,14 @@ find_ruby() {
   fi
 }
 
-unusable_ruby() {
-  if [[ -n "$HOMEBREW_MACOS_SYSTEM_RUBY_NEW_ENOUGH" ]]
+need_vendored_ruby() {
+  if [[ -n "$HOMEBREW_FORCE_VENDOR_RUBY" ]]
+  then
+    return 0
+  elif [[ -n "$HOMEBREW_MACOS_SYSTEM_RUBY_NEW_ENOUGH" ]]
   then
     return 1
-  elif [[ -z "$HOMEBREW_MACOS" && -n "$HOMEBREW_RUBY_PATH" && -z "$HOMEBREW_FORCE_VENDOR_RUBY" ]] &&
-       test_ruby "$HOMEBREW_RUBY_PATH"
+  elif [[ -z "$HOMEBREW_MACOS" ]] && test_ruby "$HOMEBREW_RUBY_PATH"
   then
     return 1
   else
@@ -93,7 +95,7 @@ If there's no Homebrew Portable Ruby available for your processor:
     fi
   else
     HOMEBREW_RUBY_PATH=$(find_ruby)
-    if [[ -z "$HOMEBREW_RUBY_PATH" || -n "$HOMEBREW_FORCE_VENDOR_RUBY" ]] || unusable_ruby
+    if need_vendored_ruby
     then
       brew vendor-install ruby || odie "$install_fail"
       HOMEBREW_RUBY_PATH="$vendor_ruby_path"