Change GitHub Actions failure mode

When a single GitHub Action build fails, the other builds should not be
canceled, but instead should continue to build.
Since the matrix is setup on OS level, a failing build on one OS usually
doesn't mean it's broken for all OS.

.github/workflows/ci.yml

@@ -8,6 +8,7 @@ jobs:
     runs-on: ${{ matrix.platform.os }}
 
     strategy:
+      fail-fast: false
       matrix:
         platform:
         - { name: Windows VS2017, os: windows-2016   }