install_arch: verify bootstrap image signature

Fixes #458 (closed).

roles/install_arch/tasks/main.yml

@@ -42,9 +42,20 @@
 
 - name: Download bootstrap image
   get_url:
-    url: https://geo.mirror.pkgbuild.com/iso/{{ bootstrap_version }}/archlinux-bootstrap-x86_64.tar.gz
+    url: "{{ item }}"
     dest: /tmp/
     mode: 0644
+  loop:
+    - https://geo.mirror.pkgbuild.com/iso/{{ bootstrap_version }}/archlinux-bootstrap-x86_64.tar.gz
+    - https://geo.mirror.pkgbuild.com/iso/{{ bootstrap_version }}/archlinux-bootstrap-x86_64.tar.gz.sig
+
+- name: Get pierre's key
+  ansible.builtin.command: gpg --locate-keys pierre@archlinux.de
+  changed_when: false
+
+- name: Get signature verification output
+  command: gpg --verify /tmp/archlinux-bootstrap-x86_64.tar.gz.sig
+  changed_when: false
 
 - name: Extract boostrap image  # noqa risky-file-permissions
   unarchive: