github(workflows): bump macOS from 11 to 12 (#597)

This bumps Clang from 13.0.0 to 13.1.6 [1][2].

`macos-12` has been available as a public beta since 2022-04-25 [3] and
exited beta on 2022-06-13 [4].

Note that `macos-latest` does not yet use macOS 12 [5].

[1] https://github.com/actions/runner-images/blob/b4c27dac058a/images/macos/macos-11-Readme.md
[2] https://github.com/actions/runner-images/blob/b4c27dac058a/images/macos/macos-12-Readme.md
[3] https://github.blog/changelog/2022-04-25-github-actions-public-beta-of-macos-12-for-github-hosted-runners-is-now-available/
[4] https://github.blog/changelog/2022-06-13-github-actions-macos-12-for-github-hosted-runners-is-now-generally-available/
[5] https://github.com/actions/runner-images/tree/b4c27dac058a#available-images

Closes: #569

.github/workflows/build.yml

@@ -24,7 +24,7 @@ jobs:
             builder: ubuntu-22.04
           - target:
               os: mac
-            builder: macos-11
+            builder: macos-12
           - target:
               os: windows
             builder: windows-2022

.github/workflows/tests.yml

@@ -29,7 +29,7 @@ jobs:
             builder: ubuntu-22.04
           - target:
               os: mac
-            builder: macos-11
+            builder: macos-12
           - target:
               os: windows
             builder: windows-2022