run tests with bundle exec

So that bigdecimal gem is found.
This fixes the following error in CI:

LoadError:
  cannot load such file -- bigdecimal
 ./lib/pg/text_encoder/numeric.rb:7:in `init_numeric'
 ./lib/pg/text_encoder/numeric.rb:7:in `<module:TextEncoder>'
 ./lib/pg/text_encoder/numeric.rb:5:in `<module:PG>'
 ./lib/pg/text_encoder/numeric.rb:4:in `<top (required)>'
 ./lib/pg/basic_type_registry.rb:229:in `register_default_types'
 ./lib/pg/basic_type_registry.rb:300:in `<class:BasicTypeRegistry>'
 ./lib/pg/basic_type_registry.rb:28:in `<top (required)>'
 ./lib/pg/basic_type_map_based_on_result.rb:57:in `<class:BasicTypeMapBasedOnResult>'
 ./lib/pg/basic_type_map_based_on_result.rb:56:in `<top (required)>'
 ./spec/pg/basic_type_map_based_on_result_spec.rb:7:in `block in <top (required)>'
 ./spec/pg/basic_type_map_based_on_result_spec.rb:6:in `<top (required)>'

.github/workflows/source-gem.yml

@@ -130,7 +130,7 @@ jobs:
       - name: Run specs
         env:
           PG_DEBUG: 0
-        run: ruby -rpg -S rspec spec/**/*_spec.rb -cfdoc
+        run: bundle exec ruby -rpg -S rspec spec/**/*_spec.rb -cfdoc
 
       - name: Print logs if job failed
         if: ${{ failure() && matrix.os == 'windows' }}