Update freebsd.yml

Author: gkorland

.github/workflows/freebsd.yml

@@ -9,8 +9,8 @@ on:
 jobs:
   build:
 
-    runs-on: macos-latest
-
+    runs-on: macos-10.15
+    
     steps:
     - uses: actions/checkout@v2
     - name: build