Make build_modes a dictionary in ibex_icache_sim_cfg.hjson

This mirrors a mistaken OpenTitan commit that I made in
December (4c89520 on the OT side), editing a vendored file. Oops!

Match that behaviour here, so that we can re-vendor without undoing
the change.

dv/uvm/icache/dv/ibex_icache_sim_cfg.hjson

@@ -29,12 +29,11 @@
 
   sim_tops: ["ibex_icache_fcov_bind"]
 
-  build_modes: [
-    {
-      name: default
+  build_modes: {
+    default: {
       en_build_modes: ["{tool}_memutil_dpi_scrambled_build_opts"]
     }
-  ]
+  }
 
   // Default iterations for all tests - each test entry can override this.
   reseed: 50