use newer version of ubuntu for compiling embedded firmware

.github/workflows/Build.yml

@@ -4,6 +4,7 @@ on:
   push:
     branches:
       - master
+      - embedded_firmware_ci
   pull_request:
     branches:
       - master
@@ -152,7 +153,7 @@ jobs:
           path: Software/PC_Application/LibreVNA-GUI.zip
           
   Embedded_Firmware:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v1
 
@@ -194,4 +195,4 @@ jobs:
           name: EmbeddedFirmware-${{env.LIBREVNA_VERSION}}
           path: |
             VNA_embedded.elf
-            combined.vnafw
+            combined.vnafw