Add workflow to ensure the gem installs fine on truffleruby

[eregon]

Follow-up of https://github.com/ruby/stringio/pull/71. As explained there we can't run the full test suite on truffleruby yet. But we can test that the gem installs fine on TruffleRuby, which is already valuable.

[kou]

How about just changing existing workflow?

diff --git a/.github/workflows/ubuntu.yml b/.github/workflows/ubuntu.yml
index 361a4eb..60557d0 100644
--- a/.github/workflows/ubuntu.yml
+++ b/.github/workflows/ubuntu.yml
@@ -42,5 +42,12 @@ jobs:
       env:
         GH_TOKEN: ${{ github.token }}
     - name: Run test
+      # TODO
+      if: >-
+        matrix.ruby != 'truffleruby-head'
       run: bundle exec rake
       continue-on-error: ${{ startsWith(matrix.ruby, 'jruby') }}
+    - name: Install gem
+      run: |
+        gem install pkg/*.gem
+        ruby -rstringio -e 'puts(StringIO::VERSION)'

[eregon]

OK, I'll do that.

[eregon]

@kou Done, and CI passing