extend/pathname: accept String as target in write_env_script

There is too much existing usage outside of Homebrew for us to break
[1].

[1]: https://github.com/search?q=%22write_env_script%20%5C%22%22%20NOT%20org%3AHomebrew%20NOT%20is%3Afork&type=code

Closes https://github.com/Homebrew/homebrew-core/pull/232573.

Library/Homebrew/extend/pathname.rb

@@ -290,7 +290,7 @@ class Pathname
 
   # Writes an exec script that sets environment variables.
   sig {
-    params(target:      Pathname,
+    params(target:      T.any(Pathname, String),
            args_or_env: T.any(String, T::Array[String], T::Hash[String, String], T::Hash[Symbol, String]),
            env:         T.any(T::Hash[String, String], T::Hash[Symbol, String])).void
   }