fix(gateway): declare hard-dependency on systemd (#10803)

Several aspects of the Gateway's Debian package depend on `systemd`
being present. Without it, we don't have the necessary users and files
in place for the Gateway to function. With that specified, we can fail
the `postinst` script (and therefore the installation) if anything in
there goes wrong.

rust/gateway/Cargo.toml

@@ -21,7 +21,7 @@ assets = [
     ["debian/firezone-gateway.sysusers", "usr/lib/sysusers.d/firezone-gateway.conf", "644"],
     ["debian/firezone-gateway.tmpfiles", "usr/lib/tmpfiles.d/firezone-gateway.conf", "644"],
 ]
-depends = 'iptables'
+depends = 'iptables,systemd'
 
 [dependencies]
 anyhow = { workspace = true }

rust/gateway/debian/postinst

@@ -1,13 +1,8 @@
 #!/usr/bin/env bash
 
-# Script inspired by deb helper script from `cargo-deb`
-if [ "$1" = "configure" ] || [ "$1" = "abort-upgrade" ] || [ "$1" = "abort-deconfigure" ] || [ "$1" = "abort-remove" ]; then
-    # In case this system is running systemd, we need to ensure that all
-    # necessary users are created before starting.
-    if [ -d /run/systemd/system ]; then
-        systemd-sysusers firezone-gateway.conf >/dev/null || true
-    fi
-fi
+set -e
+
+systemd-sysusers firezone-gateway.conf
 
 #DEBHELPER#