github-ci: pin macos build to 10.15

There is currently a build failure with macos-latest (recently updated)
to 11 in the libhtp test suite code. Not sure if there are other
build issues in libhtp or Suricata at this time.

.github/workflows/builds.yml

@@ -1312,7 +1312,8 @@ jobs:
 
   macos-latest:
     name: MacOS Latest
-    runs-on: macos-latest
+    # use 10.15 for now. Build fails on macos-11 (aka macos-latest)
+    runs-on: macos-10.15
     needs: [prepare-deps]
     steps:
       # Cache Rust stuff.