CI v3: install libgtk-3-dev (atk-sys) - gtk is an unconditional linux target dependency

.gitea/workflows/build.yml

@@ -1,4 +1,4 @@
-name: build
+name: build
 on: [push]
 
 jobs:
@@ -9,6 +9,9 @@ jobs:
       - name: Checkout
         run: git clone --depth 1 ${{ github.server_url }}/${{ github.repository }}.git .
 
+      - name: System deps (gtk for the unconditional linux target dep)
+        run: apt-get update -qq && apt-get install -y -qq libgtk-3-dev
+
       - name: Build
         run: cargo build --workspace