ci: Temporarily skip building the base variant

The Fedora 40 cloud images are currently limited in disk space in
SoftwareFactory. Let's only check for dependencies for now until this is
fixed.

See: https://pagure.io/fedora-ci/general/issue/488

ci/validate.yaml

@@ -86,10 +86,15 @@
       # Still run the next step if this one fails
       ignore_errors: true
 
-    - name: Compose the base variant
-      ansible.builtin.command:
+    - name: Perform dependency resolution for the base variant
+      ansible.builtin.shell:
         chdir: "{{ zuul.project.src_dir }}"
-        cmd: just compose-image base-atomic
+        cmd: "just compose-dry-run base-atomic"
+
+    # - name: Compose the base variant
+    #   ansible.builtin.command:
+    #     chdir: "{{ zuul.project.src_dir }}"
+    #     cmd: just compose-image base-atomic
 
     - name: Check if any previous dependency resolution steps failed
       ansible.builtin.shell: