ci: Always build Ninja with Unix Makefiles generator

The build for Ninja should not be affected by the CMAKE_GENERATOR
environment variable.

.gitlab/ci/ninja-nightly.sh

@@ -5,7 +5,7 @@ set -e
 cd .gitlab
 
 git clone https://github.com/ninja-build/ninja.git ninja-src
-cmake -S ninja-src -B ninja-src/build -DCMAKE_BUILD_TYPE=Release
+cmake -S ninja-src -B ninja-src/build -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release
 cmake --build ninja-src/build --parallel --target ninja
 mv ninja-src/build/ninja .
 rm -rf ninja-src