Give each Molecule scenario its own Ansible home

Scenarios install their Galaxy dependencies with `force: true`, so two roles
running at once re-extract the same collections and roles into ~/.ansible and
pull them out from under each other mid-play. It surfaces as a collection that
was working moments earlier going missing:

  the connection plugin 'community.docker.docker' was not found

Found while running five scenarios in parallel, where it cost a run.

ANSIBLE_HOME relocates both `collections/` and `roles/`, so one variable covers
both halves; the scenarios' ANSIBLE_ROLES_PATH workaround now follows it rather
than hardcoding ~/.ansible/roles. Left alone if already set, and unset in CI,
where each role runs in its own job and has nothing to collide with.

Verified by removing var/molecule-ansible-home entirely and running
matrix-alertmanager-receiver from cold: green through idempotence, with the
collections and roles landing under the per-role directory - which also shows
nothing was quietly relying on the shared ~/.ansible being populated.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SEH3vxYSQ5SV4N5z61eyGT

roles/custom/matrix-bridge-hookshot/molecule/default/molecule.yml

@@ -105,7 +105,7 @@ provisioner:
         matrix_bridge_hookshot_identifier: matrix-hookshot
   env:
     # Workaround for https://github.com/ansible/molecule/issues/4391
-    ANSIBLE_ROLES_PATH: ${MOLECULE_PROJECT_DIRECTORY}/../..:/.ansible/roles:/usr/share/ansible/roles:/etc/ansible/roles:~/.ansible/roles
+    ANSIBLE_ROLES_PATH: ${MOLECULE_PROJECT_DIRECTORY}/../..:/.ansible/roles:/usr/share/ansible/roles:/etc/ansible/roles:${ANSIBLE_HOME:-~/.ansible}/roles
 scenario:
   test_sequence:
     - dependency