Try to convince github actions to build all jobs instead of stopping on first fail. Using "fail-fast: false" this time

.github/workflows/linux.yml

@@ -6,6 +6,7 @@ jobs:
   build:
     runs-on: ubuntu-18.04
     strategy:
+      fail-fast: false
       matrix:
         cxx: [g++-4.8, g++-8, g++-10, clang++-9]
         build_type: [Debug, Release]

.github/workflows/macos.yml

@@ -6,6 +6,7 @@ jobs:
   build:
     runs-on: macos-10.15
     strategy:
+      fail-fast: false
       matrix:
         build_type: [Debug, Release]
 

.github/workflows/windows.yml

@@ -6,6 +6,7 @@ jobs:
   build:
     runs-on: ${{matrix.os}}
     strategy:
+      fail-fast: false
       matrix:
         # windows-2016 and windows-2019 have MSVC 2017 and 2019 installed
         # respectively: https://github.com/actions/virtual-environments.