Update build.yml

Author: mayeut

.github/workflows/build.yml

@@ -129,7 +129,7 @@ jobs:
           path: sdist
 
       - name: Ensure Ninja not present on the system
-        run: if which ninja; false; fi
+        run: if which ninja then; false; fi
 
       - name: Install SDist
         run: pip install --no-binary=ninja $(ls sdist/*.tar.gz)[test] -Ccmake.define.BUILD_CMAKE_FROM_SOURCE=OFF