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.

(cherry picked from commit 8b9721b)

.github/workflows/builds.yml

@@ -1130,7 +1130,8 @@ 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.