Update deb build container to ubuntu:20.04

.github/workflows/debian.yml

@@ -17,7 +17,7 @@ jobs:
   build:
     name: Build Kernel
     runs-on: ubuntu-latest
-    container: debian:sid
+    container: ubuntu:20.04
     steps:
     - name: Checkout code
       uses: actions/checkout@v2

.github/workflows/debian_lts.yml

@@ -17,7 +17,7 @@ jobs:
   build:
     name: Build Kernel
     runs-on: ubuntu-latest
-    container: debian:sid
+    container: ubuntu:20.04
     steps:
     - name: Checkout code
       uses: actions/checkout@v2