ci: De-duplicate Windows test script for MSVC v71 NMake job

Move job-specific setup to an environment script.

.gitlab-ci.yml

@@ -1195,7 +1195,7 @@ t:mingw_osdn_io-msys_makefiles:
 t:windows-msvc-v71-nmake:
     extends:
         - .windows_msvc_v71_nmake
-        - .cmake_test_windows_msvc
+        - .cmake_test_windows_external
         - .windows_x86_64_tags_concurrent
         - .cmake_junit_artifacts
         - .run_dependent

.gitlab/ci/env_windows_msvc_v71_nmake.ps1

@@ -0,0 +1,2 @@
+Invoke-Expression -Command .gitlab/ci/msvc.ps1
+Invoke-Expression -Command .gitlab/ci/vcvarsall.ps1

.gitlab/os-windows.yml

@@ -420,14 +420,3 @@
         - build/install/bin/ctest --output-on-failure -V -S .gitlab/ci/ctest_standalone.cmake
 
     interruptible: true
-
-.cmake_test_windows_msvc:
-    stage: test-ext
-
-    script:
-        - . .gitlab/ci/env.ps1
-        - Invoke-Expression -Command .gitlab/ci/msvc.ps1
-        - Invoke-Expression -Command .gitlab/ci/vcvarsall.ps1
-        - build/install/bin/ctest --output-on-failure -V -S .gitlab/ci/ctest_standalone.cmake
-
-    interruptible: true