#107 Reactivate build-m2 action for macOS (#145)

- Update to macOS jobs to macos-11 and reactivate macOS m2 job.
- Deactivate  ubuntu-18.04 builds. The Jenkins CI jobs already build & execute the tests against a Linux OS.
So the ubtuntu jobs are redundant and do not provide any benefit. They also increase the change of flaky or inconsistent build (e.g. ubuntu job fails but jenkins job succeeds or vice versa). 

Closes #107

Author: tortmayr

.github/workflows/build.yml

@@ -8,7 +8,7 @@ jobs:
       # do not cancel other builds if one fails
       fail-fast: false
       matrix:
-        os: [windows-2019, ubuntu-18.04]
+        os: [windows-2019, macos-11]
 
     runs-on: ${{ matrix.os }}
     timeout-minutes: 20
@@ -30,7 +30,7 @@ jobs:
       # do not cancel other builds if one fails
       fail-fast: false
       matrix:
-        os: [windows-2019, ubuntu-18.04, macos-10.15]
+        os: [windows-2019, macos-11]
 
     runs-on: ${{ matrix.os }}
     timeout-minutes: 20