feat: pin ZMK version to v0.2.1

Author: raeedcho

.github/workflows/build.yml

@@ -4,4 +4,4 @@ on: [push, pull_request, workflow_dispatch]
 
 jobs:
   build:
-    uses: zmkfirmware/zmk/.github/workflows/build-user-config.yml@main
+    uses: zmkfirmware/zmk/.github/workflows/build-user-config.yml@v0.2.1

config/west.yml

@@ -5,7 +5,7 @@ manifest:
   projects:
     - name: zmk
       remote: zmkfirmware
-      revision: main
+      revision: v0.2.1
       import: app/west.yml
   self:
     path: config