github: force Ubuntu 20.04

Apparently, ubuntu-latest still points to Ubuntu 18.04, which uses an older
and incompatible version of gcc-arm-none-eabi.

Signed-off-by: Álvaro Fernández Rojas <[email protected]>

Author: Noltari

.github/workflows/ci.yml

@@ -9,8 +9,8 @@ env:
 
 jobs:
   pico:
-    name: Raspberry Pi Pico compilation
-    runs-on: ubuntu-latest
+    name: RPi Pico compilation
+    runs-on: ubuntu-20.04
     steps:
       - name: Check out code from GitHub
         uses: actions/checkout@v2