ENV.rb: Always return integers from make_jobs

When referencing `Hardware.processor_count`, `ENV.make_jobs` will return an
integer. If referencing the environment variable `HOMEBREW_MAKE_JOBS`, it
returned a string.

Now, the function always returns an integer.

Fixes Homebrew/homebrew#12033.

Library/Homebrew/extend/ENV.rb

@@ -432,7 +432,7 @@ Please take one of the following actions:
   def make_jobs
     # '-j' requires a positive integral argument
     if self['HOMEBREW_MAKE_JOBS'].to_i > 0
-      self['HOMEBREW_MAKE_JOBS']
+      self['HOMEBREW_MAKE_JOBS'].to_i
     else
       Hardware.processor_count
     end