audit: stop red-flagging devel-only

Currently the bot is failing certain devel-only formulae because it
thinks having ` devel ` defined with an added ` head ` defined
as well = a head-only formula.

```
==> audit problems
docker-machine:
 * Head-only (no stable download)
```

This is a pretty simple fix for that problem:

```
==> brew style docker-machine
1 file inspected, no offenses detected
```

Closes #36197.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/Library/Homebrew/cmd/audit.rb b/Library/Homebrew/cmd/audit.rb
index 648ea0490..4c9896569 100644
--- a/Library/Homebrew/cmd/audit.rb
+++ b/Library/Homebrew/cmd/audit.rb
@@ -359,6 +359,10 @@ class FormulaAuditor
       problem "Head-only (no stable download)"
     end
 
+    if devel_only?(formula) && formula.tap != "homebrew/homebrew-devel-only"
+      problem "Devel-only (no stable download)"
+    end
+
     %w[Stable Devel HEAD].each do |name|
       next unless spec = formula.send(name.downcase)
 
@@ -696,7 +700,11 @@ class FormulaAuditor
   end
 
   def head_only?(formula)
-    formula.head && formula.stable.nil?
+    formula.head && formula.devel.nil? && formula.stable.nil?
+  end
+
+  def devel_only?(formula)
+    formula.devel && formula.stable.nil?
   end
 end