install_arch: verify bootstrap image signature

Fixes #458 (closed).

roles/install_arch/tasks/main.yml

@@ -54,7 +54,7 @@
   changed_when: false
 
 - name: Get signature verification output
-  command: gpg --verify archlinux-bootstrap-x86_64.tar.gz.sig
+  command: gpg --verify /tmp/archlinux-bootstrap-x86_64.tar.gz.sig
   changed_when: false
 
 - name: Extract boostrap image  # noqa risky-file-permissions