xdg-dirs: populate env vars at login

otherwise i believe they dont actually take effect?

hosts/common/home/xdg-dirs.nix

@@ -18,4 +18,12 @@
   # prevent `xdg-user-dirs-update` from overriding/updating our config
   # see <https://manpages.ubuntu.com/manpages/bionic/man5/user-dirs.conf.5.html>
   sane.user.fs.".config/user-dirs.conf".symlink.text = "enabled=False";
+
+  sane.user.fs.".profile".symlink.text = ''
+    # configure XDG_<type>_DIR preferences (e.g. for downloads, screenshots, etc)
+    # surround with `set -o allexport` since user-dirs.dirs doesn't `export` its vars
+    set -a
+    source $HOME/.config/user-dirs.dirs
+    set +a
+  '';
 }