github: switch to ubuntu-20.04

As outlined in the official blog, the 18.04 is deprecated and will be
removed soon.

https://github.blog/changelog/2022-08-09-github-actions-the-ubuntu-18-04-actions-runner-image-is-being-deprecated-and-will-be-removed-by-12-1-22/

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

.github/workflows/build-package.yml

@@ -10,8 +10,7 @@ on:
 
 jobs:
   build:
-    #runs-on: ubuntu-latest
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     steps:
       - uses: actions/checkout@v2