Fix the build on Ubuntu 20.04 (#31019)

Ubuntu 20.04 doesn't have a new enough version of GStreamer, so
automatically disable media when running on that platform.

This also cleans up the media detection a bit, putting the result in a
`enable-media` variable and moving some of the logic into the build
scripts themselves rather than the platform module.

python/servo/testing_commands.py

@@ -258,9 +258,7 @@ class MachCommands(CommandBase):
         if nocapture:
             args += ["--", "--nocapture"]
 
-        # We are setting is_build here to true, because running `cargo test` can trigger builds.
-        env = self.build_env(is_build=True)
-
+        env = self.build_env()
         return self.run_cargo_build_like_command(
             "bench" if bench else "test",
             args,