gitlab-ci: Try only building Windows in the quick flavour

It seems no matter how many machines I throw at Windows it's constantly behind.
Perhaps the quick build flavour will be fast enough to allow us to keep until
while we sort out our toolchain issues (#16084).

.gitlab-ci.yml

@@ -362,6 +362,7 @@ validate-x86_64-windows:
       set MSYSTEM=MINGW64
       python boot
       bash -c './configure --with-ghc=`pwd`/toolchain/bin/ghc --enable-tarballs-autodownload HappyCmd=`pwd`/toolchain/bin/happy AlexCmd=`pwd`/toolchain/bin/alex'
+    - bash -c "echo include mk/flavours/quick.mk > mk/build.mk"
     - bash -c "PATH=`pwd`/toolchain/bin:$PATH make -j`mk/detect-cpu-count.sh`"
     - bash -c 'make V=0 test THREADS=`mk/detect-cpu-count.sh` JUNIT_FILE=../../junit.xml'
   cache: