pull: insert questionable syntax hack

I'm not completely sure this is "sane" logic but I'm leery of just reverting
Andrew's work this morning and making him rebuild that PR from scratch for one
syntax issue.

Sadly, because we run:
```
brew readall --aliases --syntax
```
On every CI job, and that flags this line previously as:
```
pull.rb:555: warning: possibly useless use of a variable in void context
```

Every single PR has "failed" since it was merged, and it's reached the point
of being a bit annoying, so let's try this.

Library/Homebrew/cmd/pull.rb

@@ -552,7 +552,7 @@ module Homebrew
         # We're in the cache; make sure to force re-download
         while true do
           begin
-            retry_count
+            retry_count = retry_count
             curl url, "-o", filename
             break
           rescue