Use matrix.os for macos workflow "runs-on"

Previously was using macos-latest regardless of the value of
matrix.os. Should now use macOS 12, 13 and Latest.

.github/workflows/macos.yaml

@@ -28,9 +28,8 @@ concurrency:
 
 jobs:
   macos:
-    name: test macos with libevent
-    runs-on: macos-latest
-
+    name: macOS with Libevent
+
     strategy:
       fail-fast: false
       matrix:
@@ -40,6 +39,8 @@ jobs:
         tls: [ 'true', 'false' ]
         def_debug: [ 'true', 'false' ]
 
+    runs-on: ${{ matrix.os }}
+
     steps:
     - name: Install dependencies (macOS)
       if: contains(matrix.os, 'macos')