gnu: webkitgtk: Explicitly use ruby-2.

So that ruby can change without affecting this package and its dependants.

* gnu/packages/webkit.scm (webkitgtk)[native-inputs]: Explicitly use ruby-2.7.

gnu/packages/webkit.scm

@@ -222,7 +222,7 @@ engine that uses Wayland for graphics output.")
            pkg-config
            python-wrapper
            gi-docgen
-           ruby
+           ruby-2.7
            unifdef))
     (propagated-inputs
      (list gtk+ libsoup))