ci(linux): size build and test parallelism to what the shared host has free

The self-hosted runners share one 32-thread host and were fixed at one job each, so
a build with many ccache misses compiled them one after another (686 misses took 37 min).
Pick jobs from the free CPUs and available memory (1 to 8, ctest at most 4) and pass a
load limit to ninja so a busy host backs off.

Co-Authored-By: Claude <noreply@anthropic.com>

.github/workflows/build.yml

@@ -300,8 +300,6 @@ jobs:
       CCACHE_COMPILERCHECK: content
       MESA_SHADER_CACHE_DIR: /var/cache/anyps5/mesa
       MESA_SHADER_CACHE_MAX_SIZE: 4G
-      CMAKE_BUILD_PARALLEL_LEVEL: 1
-      CTEST_PARALLEL_LEVEL: 1
     steps:
       - uses: actions/checkout@v7
         with:
@@ -335,11 +333,23 @@ jobs:
           ccache --show-config
           dpkg-query -W mesa-vulkan-drivers
           grep -m1 '^model name' /proc/cpuinfo
+      - name: Size parallelism to what the shared host has free
+        run: |
+          cpus=$(nproc)
+          busy=$(awk '{ printf "%d", $1 + 0.5 }' /proc/loadavg)
+          mem_gb=$(awk '/^MemAvailable:/ { print int($2 / 1048576) }' /proc/meminfo)
+          jobs=$(( cpus - busy ))
+          jobs=$(( jobs < mem_gb / 3 ? jobs : mem_gb / 3 ))
+          jobs=$(( jobs > 8 ? 8 : jobs ))
+          jobs=$(( jobs < 1 ? 1 : jobs ))
+          echo "cpus=$cpus load=$busy mem_available_gb=$mem_gb -> build jobs $jobs"
+          echo "CMAKE_BUILD_PARALLEL_LEVEL=$jobs" >> "$GITHUB_ENV"
+          echo "CTEST_PARALLEL_LEVEL=$(( jobs > 4 ? 4 : jobs ))" >> "$GITHUB_ENV"
       - run: cmake -S . -B build $CONFIGURE -DANYPS5_ENABLE_SPIRV_TOOLS=ON
       - name: Check that every test executable is registered with ctest
         run: python3 tools/check_test_registration.py build
-      - run: cmake --build build
-      - run: cmake --build build --target libs
+      - run: cmake --build build -- -l "$(nproc)"
+      - run: cmake --build build --target libs -- -l "$(nproc)"
       - run: ctest --test-dir build --output-on-failure --timeout 120 --output-log ctest.log --output-junit ctest.xml
         env:
           ANYPS5_REQUIRE_VULKAN: 1