[rubygems/rubygems] Fix test errors when cargo not present

Currently our tests try to detect whether `cargo` is installed or not,
and if not, set tests that need `cargo` as pending.

However, when this happens that test `setup` method is completely
skipped, meaning that the `teardown` method will blow up when trying to
switch back to the original folder, since it was not set.

This commit fixes that.

https://github.com/rubygems/rubygems/commit/1e4c1e6492

test/rubygems/test_gem_ext_cargo_builder.rb

@@ -11,10 +11,10 @@ class TestGemExtCargoBuilder < Gem::TestCase
       'RUSTUP_HOME' => File.join(@orig_env['HOME'], '.rustup'),
     }
 
+    super
+
     system(@rust_envs, 'cargo', '-V', out: IO::NULL, err: [:child, :out])
     pend 'cargo not present' unless $?.success?
-
-    super
   end
 
   def setup_rust_gem(name)