Use matrix.entry for the Ubuntu workflow (#7205)

This seems easier to understand.

Co-authored-by: Nobuyoshi Nakada <nobu@ruby-lang.org>

Author: k0kubun

.github/workflows/ubuntu.yml

@@ -26,18 +26,12 @@ jobs:
   make:
     strategy:
       matrix:
-        # main variables included in the job name
-        test_task: [check]
-        configure: [cppflags=-DRUBY_DEBUG] # default to use more assertions
-        arch: ['']
-        # specify all jobs with `include` to avoid testing duplicated things
-        include:
+        entry:
           - test_task: check
           - test_task: check
             arch: i686
-            configure: '' # test without -DRUBY_DEBUG as well
           - test_task: check
-            configure: "--enable-shared --enable-load-relative"
+            configure: '--enable-shared --enable-load-relative'
           - test_task: test-all TESTS=--repeat-count=2
           - test_task: test-bundler-parallel
           - test_task: test-bundled-gems
@@ -89,8 +83,9 @@ jobs:
       - name: Run configure
         env:
           arch: ${{matrix.arch}}
+          configure: ${{matrix.configure}}
         run: >-
-          $SETARCH ../src/configure -C --disable-install-doc ${{ matrix.configure }}
+          $SETARCH ../src/configure -C --disable-install-doc ${configure:-cppflags=-DRUBY_DEBUG}
           ${arch:+--target=$arch-$OSTYPE --host=$arch-$OSTYPE}
       - run: $SETARCH make incs
       - run: $SETARCH make prepare-gems