Try to pick user login shell instead of assuming bash

The trick is inspired by `gnome-session`.

resources/niri-session

@@ -1,4 +1,15 @@
-#!/bin/bash --login
+#!/bin/sh
+
+if [ -n "$SHELL" ] &&
+   grep -q "$SHELL" /etc/shells &&
+   ! (echo "$SHELL" | grep -q "false") &&
+   ! (echo "$SHELL" | grep -q "nologin"); then
+  if [ "$1" != '-l' ]; then
+    exec bash -c "exec -l '$SHELL' -c '$0 -l $*'"
+  else
+    shift
+  fi
+fi
 
 # Make sure there's no already running session.
 if systemctl --user -q is-active niri.service; then