scripts/installer.sh: add Debian Sid (rolling release)

There is no VERSION_ID.

root@sid:~# cat /etc/os-release
PRETTY_NAME="Debian GNU/Linux bookworm/sid"
NAME="Debian GNU/Linux"
VERSION_CODENAME=bookworm
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"

Fixes #5522

Signed-off-by: Denton Gentry <dgentry@tailscale.com>

scripts/installer.sh

@@ -66,7 +66,10 @@ main() {
 				PACKAGETYPE="apt"
 				# Third-party keyrings became the preferred method of
 				# installation in Debian 11 (Bullseye).
-				if [ "$VERSION_ID" -lt 11 ]; then
+				if [ -z "${VERSION_ID:-}" ]; then
+					# rolling release. If you haven't kept current, that's on you.
+					APT_KEY_TYPE="keyring"
+				elif [ "$VERSION_ID" -lt 11 ]; then
 					APT_KEY_TYPE="legacy"
 				else
 					APT_KEY_TYPE="keyring"