Add beta versions of runners to CI build

This change adds support for the public beta of Ubuntu 22.04 and
macOS 12. As this is a beta, this change also updates the
continue-on-error field to indicate that a build failure on the 22.04
runner should not cause the rest of the build to fail.

[skip doxy]

.github/workflows/build.yml

@@ -15,13 +15,13 @@ jobs:
     if: "!(contains(github.event.head_commit.message, '[ci skip]') || contains(github.event.head_commit.message, '[skip ci]'))"
     name: ${{ join(matrix.*, '/') }}
     runs-on: ${{ matrix.os }}
-    continue-on-error: ${{ startsWith(matrix.os, 'macos-') }}
+    continue-on-error: ${{ startsWith(matrix.os, 'macos-') || (matrix.os == 'ubuntu-22.04') }}
     env:
       CC: ${{ matrix.cc }}
 
     strategy:
       matrix:
-        os: ['ubuntu-18.04', 'ubuntu-20.04', 'macos-10.15', 'macos-11']
+        os: ['ubuntu-18.04', 'ubuntu-20.04', 'ubuntu-22.04', 'macos-10.15', 'macos-11', 'macos-12']
         cc: ['gcc', 'clang']
 
     steps: