ci: GitLab -j6 (#3677)

Tune down number of jobs in GitLab back to 6. I suspect our current heaviest hitter, the GX2F test, is causing OOMs in some cases. Until that's fixed, we need to reduce the core count again.

.gitlab-ci.yml

@@ -81,7 +81,7 @@ build_exatrkx_cpu:
       -DACTS_EXATRKX_ENABLE_CUDA=OFF
 
     - ccache -z
-    - cmake --build build -- -j8
+    - cmake --build build -- -j6
     - ccache -s
 
 build_exatrkx:
@@ -119,7 +119,7 @@ build_exatrkx:
       -DCMAKE_CUDA_ARCHITECTURES="75;86"
 
     - ccache -z
-    - cmake --build build -- -j8
+    - cmake --build build -- -j6
     - ccache -s
 
 # test_exatrkx_unittests:
@@ -186,7 +186,7 @@ build_linux_ubuntu:
       -DACTS_BUILD_PLUGIN_ONNX=ON
 
     - ccache -z
-    - cmake --build build -- -j8
+    - cmake --build build -- -j6
     - ccache -s
 
 linux_test_examples:
@@ -271,7 +271,7 @@ linux_physmon:
       -DCMAKE_CXX_STANDARD=${CXXSTD}
 
     - ccache -z
-    - cmake --build build -- -j8
+    - cmake --build build -- -j6
     - ccache -s
 
     - ctest --test-dir build -j$(nproc)
@@ -367,7 +367,7 @@ linux_ubuntu_2204_clang:
       -DACTS_BUILD_PLUGIN_GEOMODEL=OFF # GeoModel is not in LCG at this point
 
     - ccache -z
-    - cmake --build build -- -j8
+    - cmake --build build -- -j6
     - ccache -s
 
     - ctest --test-dir build -j$(nproc)