formula_assertions: Fix type of cmd param in shell_output

- This can be either a String or a Pathname, per
  the part of the `noseyparker` test that failed
  (in a different part of the test, the command is
  passed as a string).

```
  ==> Testing noseyparker
  ==> /opt/homebrew/Cellar/noseyparker/0.18.1/bin/noseyparker -V
  Error: noseyparker: failed
  An exception occurred within a child process:
    TypeError: Parameter 'cmd': Expected type String, got type Pathname with value #<Pathname:/opt/homebrew/Ce...ps://github.com/Homebrew/brew>
  Caller: /opt/homebrew/Library/Taps/homebrew/homebrew-core/Formula/n/noseyparker.rb:35
```

Library/Homebrew/formula_assertions.rb

@@ -21,7 +21,7 @@ module Homebrew
     # Returns the output of running cmd and asserts the exit status.
     #
     # @api public
-    sig { params(cmd: String, result: Integer).returns(String) }
+    sig { params(cmd: T.any(Pathname, String), result: Integer).returns(String) }
     def shell_output(cmd, result = 0)
       ohai cmd
       output = `#{cmd}`