Drop custom repo from libuser install @ Alpine CI (#78305)

`group` and `setup_cron` integration tests currently install `libuser` and `faketime`
packages respectively, from the non-default edge/testing repositories. It is no
longer necessary to do so for the former so this patch drops the repo option. As
for the latter, this patch includes a note explaining when the same could be done
for it too.

This patch also adds a note on installing `faketime` on Alpine in CI.

Co-Authored-By: Matt Clay <matt@mystile.com>

test/integration/targets/group/tasks/tests.yml

@@ -211,7 +211,7 @@
     - user_test_local_mode
 
 - name: Ensure lgroupadd is present - Alpine
-  command: apk add -U libuser --repository=http://dl-cdn.alpinelinux.org/alpine/edge/community
+  command: apk add -U libuser
   when: ansible_distribution == 'Alpine'
   tags:
     - user_test_local_mode

test/integration/targets/setup_cron/tasks/main.yml

@@ -27,6 +27,11 @@
       when: ansible_distribution != 'Alpine'
 
     - name: install faketime packages - Alpine
+      # NOTE: The `faketime` package is currently only available in the
+      # NOTE: `edge` branch.
+      # FIXME: If it ever becomes available in the `main` repository for
+      # FIXME: currently tested Alpine versions, the `--repository=...`
+      # FIXME: option can be dropped.
       command: apk add -U {{ faketime_pkg }} --repository=http://dl-cdn.alpinelinux.org/alpine/edge/testing
       when: ansible_distribution == 'Alpine'