firstboot: validate root shell credentials

Validate passwd.shell.root after reading the credential so it uses
the same target-root shell checks as --root-shell= and prompted input.

Reproducer: CREDENTIALS_DIRECTORY=... systemd-firstboot --root=...
with passwd.shell.root=/bin/nonexistentshell

Before: /etc/passwd was written with the nonexistent root shell and
the command exited successfully.

Follow-up: 416f7b3a11

src/firstboot/firstboot.c

@@ -998,6 +998,10 @@ static int prompt_root_shell(int rfd, sd_varlink **mute_console_link) {
         if (r < 0)
                 log_debug_errno(r, "Failed to read credential passwd.shell.root, ignoring: %m");
         else {
+                r = find_shell(rfd, arg_root_shell);
+                if (r < 0)
+                        return r;
+
                 log_debug("Acquired root shell from credential.");
                 return 0;
         }