Switch the github macOS CI over to 12.4, which is current.

Needed because the build now relies on `readlink -f`, but I don't
think we really care about old versions of macOS anyway, since Mac
users upgrade pretty quickly? It's a bit odd that the _current_
version wasn't being tested in CI, but macOS 10.15 did still make
some sense because it's the oldest version that's still getting
security updates.

.github/workflows/toybox.yml

@@ -7,22 +7,8 @@ on:
     branches: [ master ]
 
 jobs:
-  MacOS-11_0:
-    runs-on: macos-11.0
-
-    steps:
-    - uses: actions/checkout@v2
-    - name: Setup
-      run: brew install gnu-sed
-    - name: Configure
-      run: make macos_defconfig
-    - name: Build
-      run: make
-    - name: Test
-      run: VERBOSE=all make tests
-
-  MacOS-10_15:
-    runs-on: macos-10.15
+  MacOS-12:
+    runs-on: macos-12.4
 
     steps:
     - uses: actions/checkout@v2