fix(schematics): toolbar in nav w/o z-index causes issues with dashboard (#13270)

Without this z-index, scrolling a dashboard causes issues
- headers and background scroll under toolbar
- cards scroll over top of the toolbar

Author: Splaktar

src/lib/schematics/nav/files/__path__/__name@dasherize@if-flat__/__name@dasherize__.component.__styleext__

@@ -9,4 +9,5 @@
 .mat-toolbar.mat-primary {
   position: sticky;
   top: 0;
+  z-index: 1;
 }