ci: Factor out scripts to add ninja to job environment

.gitlab/ci/ninja-env.sh

@@ -0,0 +1,3 @@
+.gitlab/ci/ninja.sh
+export PATH=$PWD/.gitlab:$PATH
+ninja --version

.gitlab/os-linux.yml

@@ -512,9 +512,7 @@
 .before_script_linux: &before_script_linux
     - source .gitlab/ci/env.sh
     - source .gitlab/ci/cmake-env.sh
-    - .gitlab/ci/ninja.sh
-    - export PATH=$PWD/.gitlab:$PATH
-    - ninja --version
+    - source .gitlab/ci/ninja-env.sh
 
 .cmake_prep_source_linux:
     stage: prep

.gitlab/os-macos.yml

@@ -176,9 +176,7 @@
 .before_script_macos: &before_script_macos
     - source .gitlab/ci/env.sh
     - source .gitlab/ci/cmake-env.sh
-    - .gitlab/ci/ninja.sh
-    - export PATH=$PWD/.gitlab:$PATH
-    - ninja --version
+    - source .gitlab/ci/ninja-env.sh
     # Download Qt
     - cmake -P .gitlab/ci/download_qt.cmake
     - export CMAKE_PREFIX_PATH=$PWD/.gitlab/qt${CMAKE_PREFIX_PATH:+:$CMAKE_PREFIX_PATH}