CI/msys2: Don't install sassc to skip CSS generation and make job errors fatal again

Once #2237 is fixed we can install sassc again, until then use the in-tree CSS.

.gitlab-ci.yml

@@ -46,7 +46,6 @@ msys2-mingw32-meson:
   script:
     - C:\msys64\usr\bin\pacman --noconfirm -Syyuu
     - C:\msys64\usr\bin\bash -lc "bash -x ./.gitlab-ci/test-msys2-meson.sh"
-  allow_failure: true
   artifacts:
     when: always
     name: "gtk3-%CI_JOB_NAME%-%CI_COMMIT_REF_NAME%"
@@ -64,4 +63,3 @@ msys2-mingw32-autotools:
   script:
     - C:\msys64\usr\bin\pacman --noconfirm -Syyuu
     - C:\msys64\usr\bin\bash -lc "bash -x ./.gitlab-ci/test-msys2-autotools.sh"
-  allow_failure: true

.gitlab-ci/test-msys2-meson.sh

@@ -28,8 +28,7 @@ pacman --noconfirm -S --needed \
     mingw-w64-$MSYS2_ARCH-shared-mime-info \
     mingw-w64-$MSYS2_ARCH-meson \
     mingw-w64-$MSYS2_ARCH-ninja \
-    mingw-w64-$MSYS2_ARCH-gtk-doc \
-    mingw-w64-$MSYS2_ARCH-sassc
+    mingw-w64-$MSYS2_ARCH-gtk-doc
 
 mkdir -p _ccache
 export CCACHE_BASEDIR="$(pwd)"