ci: Simplify LLVM/Clang CI job specs on Windows

Move environment setup to job-specific scripts so we can reuse
the common `.cmake_test_windows_external` code path.

.gitlab-ci.yml

@@ -1003,7 +1003,7 @@ t:windows-borland5.8:
 t:windows-clang13.0-cl-ninja:
     extends:
         - .windows_clang_ninja
-        - .cmake_test_windows_clang
+        - .cmake_test_windows_external
         - .windows_tags_concurrent
         - .cmake_junit_artifacts
         - .run_dependent
@@ -1018,7 +1018,7 @@ t:windows-clang13.0-cl-ninja:
 t:windows-clang13.0-cl-nmake:
     extends:
         - .windows_clang_nmake
-        - .cmake_test_windows_clang
+        - .cmake_test_windows_external
         - .windows_tags_concurrent
         - .cmake_junit_artifacts
         - .run_dependent
@@ -1033,7 +1033,7 @@ t:windows-clang13.0-cl-nmake:
 t:windows-clang13.0-gnu-ninja:
     extends:
         - .windows_clang_ninja
-        - .cmake_test_windows_clang
+        - .cmake_test_windows_external
         - .windows_tags_concurrent
         - .cmake_junit_artifacts
         - .run_dependent
@@ -1048,7 +1048,7 @@ t:windows-clang13.0-gnu-ninja:
 t:windows-clang13.0-gnu-nmake:
     extends:
         - .windows_clang_nmake
-        - .cmake_test_windows_clang
+        - .cmake_test_windows_external
         - .windows_tags_concurrent
         - .cmake_junit_artifacts
         - .run_dependent

.gitlab/ci/env_windows_clang_ninja.ps1

@@ -0,0 +1,2 @@
+. .gitlab/ci/ninja-env.ps1
+. .gitlab/ci/clang-env.ps1

.gitlab/ci/env_windows_clang_nmake.ps1

@@ -0,0 +1 @@
+. .gitlab/ci/clang-env.ps1

.gitlab/os-windows.yml

@@ -390,17 +390,6 @@
 
     interruptible: true
 
-.cmake_test_windows_clang:
-    stage: test-ext
-
-    script:
-        - . .gitlab/ci/env.ps1
-        - . .gitlab/ci/ninja-env.ps1
-        - . .gitlab/ci/clang-env.ps1
-        - build/install/bin/ctest --output-on-failure -V -S .gitlab/ci/ctest_standalone.cmake
-
-    interruptible: true
-
 .cmake_test_windows_msvc:
     stage: test-ext