[github-actions] fix macOS builds (#7086)

This commit fixes macOS build failures by downgrading to macOS 10.15.

.github/workflows/build.yml

@@ -279,7 +279,7 @@ jobs:
             CXX: clang++
           - CC: gcc
             CXX: g++
-    runs-on: macos-11
+    runs-on: macos-10.15
     env:
       CC: ${{ matrix.CC }}
       CXX: ${{ matrix.CXX }}

.github/workflows/posix.yml

@@ -225,7 +225,7 @@ jobs:
 
   pty-macos:
     name: pty-macos OT_DAEMON=${{ matrix.OT_DAEMON }}
-    runs-on: macos-11
+    runs-on: macos-10.15
     strategy:
       matrix:
         OT_DAEMON: ['off', 'on']