build: break `ninja && ninja install` into 2 calls.

Fixing a case where the script continued after a failed `ninja`. After further
testing on how `set -e` works, it looks like a toplevel `false && true` command
will fail the script (which is what we want), but it won't when inside an `if`
block!

Breaking it down into 2 separate calls fail the script correctly though. So
let's do this.

build/windows/gitlab-ci/2_build-gimp-msys2.sh

@@ -98,10 +98,12 @@ if [ ! -f "_build${ARTIFACTS_SUFFIX}/build.ninja" ]; then
                  -Djavascript=disabled               \
                  -Ddirectx-sdk-dir="${MSYS2_PREFIX}" \
                  -Dbuild-id=org.gimp.GIMP_official
-  ninja && ninja install
+  ninja
+  ninja install
 else
   cd "_build${ARTIFACTS_SUFFIX}"
-  ninja && ninja install
+  ninja
+  ninja install
 fi