Couple of debian fixes, cannot use variable expansions this way

debian/bird-exporter.postinst

@@ -0,0 +1,18 @@
+#!/bin/sh
+set -e
+
+case "$1" in
+    configure)
+        # Create bird user if it doesn't exist
+        if ! getent passwd bird > /dev/null; then
+            adduser --system --group --no-create-home \
+                --home /nonexistent \
+                --gecos "BIRD routing daemon" \
+                bird
+        fi
+        ;;
+esac
+
+#DEBHELPER#
+
+exit 0