ci(build): remove artifact uf2 suffix

The archive now contains hex and uf2 files so the uf2 suffix is no longer accurate.  It probably should've been removed as part of the earlier hex commit.

See: 97d045e7ef
PR: #496

.github/workflows/build.yml

@@ -111,7 +111,7 @@ jobs:
       - name: Archive build
         uses: actions/upload-artifact@v2
         with:
-          name: "${{ steps.variables.outputs.artifact-name }}-uf2"
+          name: "${{ steps.variables.outputs.artifact-name }}"
           path: |
             build/zephyr/zmk.hex
             build/zephyr/zmk.uf2