formula_info: use RUBY_PATH instead of calling HOMEBREW_BREW_FILE

This ensures correct RUBY_PATH is used when running `brew tests` with
custom `HOMEBREW_RUBY_PATH`

More info: https://github.com/Homebrew/brew/pull/6617#discussion_r336431131

Library/Homebrew/formula_info.rb

@@ -13,7 +13,15 @@ class FormulaInfo
   # Looks up formula on disk and reads its info.
   # Returns nil if formula is absent or if there was an error reading it.
   def self.lookup(name)
-    json = Utils.popen_read(HOMEBREW_BREW_FILE, "info", "--json=v1", name)
+    json = Utils.popen_read(
+      RUBY_PATH,
+      "-W0",
+      "-I", $LOAD_PATH.join(File::PATH_SEPARATOR),
+      HOMEBREW_LIBRARY_PATH/"brew.rb",
+      "info",
+      "--json=v1",
+      name
+    )
 
     return unless $CHILD_STATUS.success?