archlinux: create user 'user' using bash by default instead of zsh

The bash/zsh bug should not be present anymore in Qubes 4.0
as discussed in the issue 2888.
(https://github.com/QubesOS/qubes-issues/issues/2888)

archlinux/PKGBUILD.install

@@ -12,7 +12,7 @@ update_default_user() {
     # Archlinux bash version has a 'bug' when running su -c, /etc/profile is not loaded because bash consider there is no interactive pty when running 'su - user -c' or something like this.
     # See https://bugs.archlinux.org/task/31831
     id -u 'user' >/dev/null 2>&1 || {
-        useradd --user-group --create-home --shell /bin/zsh user
+        useradd --user-group --create-home --shell /bin/bash user
     }
     usermod -a --groups qubes user
 }