Fix removing old built opencv_ffmpeg when a new upstream formula is available

Happened in https://travis-ci.org/skvark/opencv-python/builds/518179810 S1 jobs

Author: native-api

config.sh

@@ -92,6 +92,7 @@ function pre_build {
     #after the cache stage, all bottles and Homebrew metadata should be already cached locally
     if [ -n "$CACHE_STAGE" ]; then
         brew update
+        generate_ffmpeg_formula
         brew_add_local_bottles
     fi
 
@@ -107,7 +108,6 @@ function pre_build {
     echo 'Installing FFmpeg'
 
     if [ -n "$CACHE_STAGE" ]; then
-        generate_ffmpeg_formula
         brew_install_and_cache_within_time_limit ffmpeg_opencv || { [ $? -gt 1 ] && return 2 || return 0; }
     else
         brew install ffmpeg_opencv