Expand JOBS instead of meaningless export (#2402)

* Expand JOBS instead of meaningless export

* Remove -j option in TESTOPTS of test-bundled-gems

https://github.com/ruby/ruby/runs/203129516#step:10:167
> invalid option: -j3

.github/workflows/ubuntu.yml

@@ -34,9 +34,6 @@ jobs:
           GITHUB_BRANCH: ${{ github.event.pull_request.head.ref }}
           GITHUB_REPO: ${{ github.event.pull_request.head.repo.full_name }}
         if: github.event_name == 'pull_request'
-      - name: Set ENV
-        run: |
-          export JOBS=-j$((1 + $(nproc --all)))
       - name: Fixed world writable dirs
         run: |
           chmod go-w $HOME
@@ -44,18 +41,17 @@ jobs:
       - run: autoconf
       - name: configure
         run: ./configure -C --disable-install-doc
-      - run: make $JOBS
+      - run: make -j$((1 + $(nproc --all)))
       - name: Tests
-        run: make -s ${{ matrix.test_task }}
+        run: make -s TESTOPTS="-j$((1 + $(nproc --all))) -q --tty=no" ${{ matrix.test_task }}
         env:
-          TESTOPTS: "$JOBS -q --tty=no"
           MSPECOPT: "-ff" # not using `-j` because sometimes `mspec -j` silently dies
         if: matrix.test_task != 'test-bundled-gems' && !contains(github.event.head_commit.message, '[ci skip]')
       # test-bundled-gems is separated for marking `continue-on-error` because it randomly fails.
       - name: Tests
         run: make -s ${{ matrix.test_task }}
         env:
-          TESTOPTS: "$JOBS -q --tty=no"
+          TESTOPTS: "-q --tty=no"
         continue-on-error: true
         if: matrix.test_task == 'test-bundled-gems' && !contains(github.event.head_commit.message, '[ci skip]')
       - name: Leaked Globals