Save pico firmware as an artifact

In the future, we could put this on the build server.

.gitlab-ci.yml

@@ -15,6 +15,12 @@ stages:
 before_script:
   - "pip install -U platformio"
 
-job:
+build:
   stage: build
-  script: "pio run -e nano -e leonardo -e pico -e bluepill -e esp32"
+  script: 
+    - "pio run -e nano -e leonardo -e pico -e bluepill -e esp32"
+    - mkdir -p builds
+    - cp .pio/build/pico/firmware.uf2 builds/
+  artifacts:
+    paths:
+      - builds/*