ci: build for p6

Also drive-by removed unnecessary git submodule crud - we can now check
out all submodules without wasting time.

.gitlab-ci.yml

-variables:
-  GIT_SUBMODULE_STRATEGY: recursive
-  GIT_SUBMODULE_PATHS: micropython/lib/berkeley-db-1.xx micropython/lib/micropython-lib
-
-
 stages:
   - build
 
@@ -35,3 +30,13 @@ build-p4:
   artifacts:
     expose_as: 'Proto 4 ELF'
     paths: ['build/badge23.elf']
+
+build-p6:
+  stage: build
+  tags:
+    - nixos
+  script:
+    - nix-shell nix/shell.nix --command "idf.py -g p6 build"
+  artifacts:
+    expose_as: 'Proto 6 ELF'
+    paths: ['build/badge23.elf']