refactor(boards): Move nice!60 to ZMK variant by default

Make the standard nice!60 board ID be `nice60/nrf52840/zmk` or
`nice60//zmk` by shorthand.

app/boards/nicekeyboards/nice60/README.md

@@ -7,5 +7,5 @@ The nice!60 is a hotswap 60% made by Nice Keyboards. https://nicekeyboards.com/n
 ## Building nice!60 ZMK firmware
 
 ```
-west build -p -b nice60
+west build -p -b nice60//zmk
 ```

app/boards/nicekeyboards/nice60/board.yml

@@ -3,3 +3,5 @@ board:
   vendor: nicekeyboards
   socs:
     - name: nrf52840
+      variants:
+        - name: zmk

app/boards/nicekeyboards/nice60/nice60.zmk.yml

@@ -1,5 +1,5 @@
 file_format: "1"
-id: nice60
+id: nice60//zmk
 name: nice!60
 type: board
 arch: arm

app/core-coverage.yml

@@ -10,7 +10,7 @@ shield:
   - tidbit
 include:
   - board: bdn9//zmk
-  - board: nice60
+  - board: nice60//zmk
   - board: xiao_ble//zmk
     shield: hummingbird
   - board: nrf52840_m2//zmk