ci: limit GCC concurrency to -j 1 on ubuntu-26.04 and fix skip_tests condition

Author: msg7086

.github/workflows/ci.yml

@@ -21,6 +21,7 @@ jobs:
             os: ubuntu-26.04
             cmake_generator: "Ninja"
             cmake_flags: "-DCMAKE_BUILD_TYPE=Release -DENABLE_PAR=OFF -DF3KDB_TEST_HOSTS=vs"
+            build_flags: "-j 1"
 
           - name: ubuntu-26.04-arm
             os: ubuntu-26.04-arm
@@ -41,13 +42,13 @@ jobs:
             os: windows-11-vs2026-arm
             cmake_generator: "Visual Studio 18 2026"
             cmake_flags: "-T ClangCL -A ARM64 -DENABLE_PAR=OFF"
-            run_tests: false
+            skip_tests: true
 
           - name: windows-11-clang-arm
             os: windows-11-vs2026-arm
             cmake_generator: "Ninja"
             cmake_flags: "-DCMAKE_BUILD_TYPE=Release -DCMAKE_C_COMPILER=clang -DCMAKE_CXX_COMPILER=clang++ -DENABLE_PAR=OFF"
-            run_tests: false
+            skip_tests: true
 
     steps:
       - name: Checkout Code
@@ -59,7 +60,7 @@ jobs:
           python-version: "3.12"
 
       - name: Setup Windows Test Environment (VS & AVS)
-        if: runner.os == 'Windows' && matrix.run_tests != false
+        if: runner.os == 'Windows' && !matrix.skip_tests
         run: |
           python -m pip install --upgrade pip
           python -m pip install vapoursynth
@@ -118,8 +119,8 @@ jobs:
           ${{ matrix.cmake_flags }}
 
       - name: Build
-        run: cmake --build build --config Release --parallel
+        run: cmake --build build --config Release ${{ matrix.build_flags || '--parallel' }}
 
       - name: Test (CTest)
-        if: matrix.run_tests != false
+        if: "!matrix.skip_tests"
         run: ctest --test-dir build -C Release --output-on-failure