Update build-machines

Author: martinweismann

.github/workflows/build.yml

@@ -2,15 +2,15 @@ on: [push, pull_request]
 name: Build
 jobs:
   build-linux:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v2
         with:
           submodules: true
       - run: ./build.sh
         working-directory: ./Build 
   build-macos:
-    runs-on: macos-10.15
+    runs-on: macos-11
     steps:
       - uses: actions/checkout@v2
         with: