Use slurp module to fetch remote file content

The slurp module will fetch as b64 the content of remote file.
Using slurp allows working in relative path like other modules, instead
of trying absolute paths in a lookup.

Change-Id: I773819d1d17fb6faaedacf73dea8f645b3cb9699

roles/charm-publish/tasks/publish.yaml

@@ -37,10 +37,17 @@
         dest: "{{ zuul.project.src_dir }}/{{ charm_build_name }}"
         remote_src: true
 
+    - name: Read metadata.yaml
+      slurp:
+        src: "{{ zuul.project.src_dir }}/{{ charm_build_name }}/metadata.yaml"
+      register: metadata_file
+
+    - name: Parse metadata.yaml
+      set_fact:
+        metadata: "{{ metadata_file.content | b64decode | from_yaml }}"
+
     - name: Upload oci-image to charmhub
       register: upload_oci_image_output
-      vars:
-        metadata: "{{ lookup('file', zuul.executor.work_root + '/' + zuul.project.src_dir + '/' + charm_build_name + '/metadata.yaml') | from_yaml }}"
       args:
         executable: /bin/bash
       shell: |