Small change in sysupgrade for custom release and test

[Sven F.]

Bienvenue,

--- /usr/sbin/sysupgrade.old    Tue May 16 18:53:13 2023
+++ /usr/sbin/sysupgrade        Tue May 16 19:04:46 2023
@@ -143,6 +143,7 @@
 case ${_LINE} in
 *\ ${_KEY})    SIGNIFY_KEY=/etc/signify/${_KEY} ;;
 *\ ${_NEXTKEY})        SIGNIFY_KEY=/etc/signify/${_NEXTKEY} ;;
+*\ *.pub)  SIGNIFY_KEY=/etc/signify/${_LINE##* } && echo Using custom
key $SIGNIFY_KEY ;;
 *)             err "invalid signing key" ;;
 esac

Read the signing key in the file so you can use a custom key when
testing release,

Have a good one.