home.pointerCursor: set common XCURSOR_* environment variables by default (#3663)

The `XCURSOR_*` environment variables specified in libxcursor
are used by many applications and libraries to load and configure
cursor settings. Setting these variables is a no-op if ignored but
is useful as a fallback when other sources of configuration are
unreliable.

This commit sets some commonly used `XCURSOR_*` environment variables
(i.e XCURSOR_THEME, XCURSOR_SIZE) by default when `home.pointerCursor`
is enabled.

modules/config/home-cursor.nix

@@ -133,6 +133,8 @@ in {
       home.sessionVariables = {
         XCURSOR_PATH = mkDefault ("$XCURSOR_PATH\${XCURSOR_PATH:+:}"
           + "${config.home.profileDirectory}/share/icons");
+        XCURSOR_SIZE = mkDefault cfg.size;
+        XCURSOR_THEME = mkDefault cfg.name;
       };
     }