workflows: enable build of the AVIF gdk-pixbuf loader

.github/workflows/ci-unix-shared-installed.yml

diff --git a/.github/workflows/ci-unix-shared-installed.yml b/.github/workflows/ci-unix-shared-installed.yml
index 31b520d..fab8389 100644
--- a/.github/workflows/ci-unix-shared-installed.yml
+++ b/.github/workflows/ci-unix-shared-installed.yml
@@ -70,6 +70,7 @@
         -DAVIF_CODEC_DAV1D=ON
         -DAVIF_BUILD_EXAMPLES=ON -DAVIF_BUILD_APPS=ON
         -DAVIF_BUILD_TESTS=ON -DAVIF_ENABLE_GTEST=ON -DAVIF_LOCAL_GTEST=ON
+        -DAVIF_BUILD_GDK_PIXBUF=ON
     - name: Build libavif (ninja)
       working-directory: ./build
       run: ninja