gitlab-ci: move macOS x86_64 jobs back to shell runners

CMake has been observing crashes under Rosetta for its x86_64 jobs.
Additionally, `tart` has significant overhead when running Rosetta
work compared to bare metal.

.gitlab/os-macos.yml

@@ -19,12 +19,26 @@
 
 ### Build and test
 
+.macos_build:
+    variables:
+        # Note that shell runners only support runners with a single
+        # concurrency level. We can't use `$CI_CONCURRENCY_ID` because this may
+        # change between the build and test stages which CMake doesn't support.
+        # Even if we could, it could change if other runners on the machine
+        # could run at the same time, so we drop it.
+        GIT_CLONE_PATH: "$CI_BUILDS_DIR/cmake ci"
+
 .macos_tart_build:
     extends:
         - .macos_tart
 
+.macos_x86_64_build:
+    extends:
+        - .macos_x86_64
+        - .macos_build
+
 .macos_x86_64_ninja:
-    extends: .macos_tart_build
+    extends: .macos_x86_64_build
 
     variables:
         CMAKE_CONFIGURATION: macos_x86_64_ninja
@@ -55,7 +69,7 @@
         CMAKE_CONFIGURATION: macos_arm64_pch
 
 .macos_x86_64_makefiles:
-    extends: .macos_tart_build
+    extends: .macos_x86_64_build
 
     variables:
         CMAKE_CONFIGURATION: macos_x86_64_makefiles
@@ -148,7 +162,9 @@
     tags:
         - cmake
         - macos-x86_64
-        - tart
+        - shell
+        - xcode-26.3
+        - nonconcurrent
 
 .macos_x86_64_tags_ext:
     tags: