Prepare cargo_embargo.json for multi-variants

Minimize the diff of the next commits by packaging the configs for the
single (for now) variant into one "variants" entry (trivial change).

Test: cargo_embargo generate cargo_embargo.json
Test: m libmemoffset
Change-Id: Idc9f5a113c8dde77047b92e6b222c35f38f41ae2
diff --git a/cargo_embargo.json b/cargo_embargo.json
index 87c3870..c458c58 100644
--- a/cargo_embargo.json
+++ b/cargo_embargo.json
@@ -1,5 +1,4 @@
 {
-  "min_sdk_version": "29",
   "package": {
     "memoffset": {
       "dep_blocklist": [
@@ -7,5 +6,10 @@
       ]
     }
   },
-  "tests": true
+  "variants": [
+    {
+      "min_sdk_version": "29",
+      "tests": true
+    }
+  ]
 }