formula: ensure rpath is passed a valid target

Setting `target` to a path outside `HOMEBREW_PREFIX` is not portable,
and will be invalid for some Homebrew installations.

If we want to add an `rpath` outside of `HOMEBREW_PREFIX`, we should
just use the absolute path instead.

See discussion at Homebrew/homebrew-core#110520.

Library/Homebrew/formula.rb

@@ -1615,6 +1615,10 @@ class Formula
   # </pre>
   sig { params(source: Pathname, target: Pathname).returns(String) }
   def rpath(source: bin, target: lib)
+    unless target.to_s.start_with?(HOMEBREW_PREFIX)
+      odie "`target` should only be used for paths inside HOMEBREW_PREFIX!"
+    end
+
     "#{loader_path}/#{target.relative_path_from(source)}"
   end