Update coverage workflow to use Clang for compilation

.github/workflows/coverage.yml

@@ -8,10 +8,11 @@ jobs:
     runs-on: ubuntu-24.04
     env:
       MRUBY_CONFIG: ci/gcc-clang
-      CC: gcc
-      CXX: g++
-      LD: gcc
+      CC: clang
+      CXX: clang++
+      LD: clang
       CFLAGS: -O0 -g --coverage
+      CXXFLAGS: -O0 -g --coverage
       LDFLAGS: --coverage
     steps:
       - name: "Checkout ${{ github.ref }} ( ${{ github.sha }} )"
@@ -29,7 +30,7 @@ jobs:
           {
             echo "# Coverage report"
             echo \`\`\`
-            gcovr -e ".*_test.c" -e ".*/test/.*" -s --html-details -o coverage/ build
+            gcovr --gcov-executable "llvm-cov gcov" -e ".*_test.c" -e ".*/test/.*" -s --html-details -o coverage/ build
             echo \`\`\`
           } > "$GITHUB_STEP_SUMMARY"
       - name: Upload coverage report