copr: be: avoid dnf warning for loops

[DEPRECATION WARNING]: Invoking "dnf" only once while using a loop via
squash_actions is deprecated. Instead of using a loop to supply multiple
items and specifying `pkg: "{{ item }}"`,

roles/copr/backend/files/provision/provision_builder_tasks.yml

@@ -38,10 +38,11 @@
     - prepare_base_image is defined
 
 - name: install copr-builder and other latest packages
-  dnf: state=latest pkg={{ item }}
-  with_items:
-  - copr-builder
-  - libcurl # temporarily, for rhbz#1690971
+  dnf: state=latest pkg="{{ packages }}"
+  vars:
+    packages:
+    - copr-builder
+    - libcurl # temporarily, for rhbz#1690971
 
 # temporary thing, till we have merged:
 # https://pagure.io/copr/copr/pull-request/1109