Fix CI for Mac OS

Build ffmpeg as external project to work around missing includes:
/Users/runner/work/sdrangel/sdrangel/plugins/channelrx/demoddatv/datvideorender.cpp:282:46: error: no member named 'channels' in 'AVCodecContext'
(and similar)

.github/workflows/sdrangel.yml

@@ -121,7 +121,7 @@ jobs:
           rm -f /usr/local/bin/2to3
           brew install opencv
       - name: Install brew packages
-        run: brew install nasm boost hidapi libusb fftw ffmpeg faad2 zlib airspy airspyhf hackrf rtl-sdr libbladerf soapysdr qt
+        run: brew install nasm boost hidapi libusb fftw faad2 zlib airspy airspyhf hackrf rtl-sdr libbladerf soapysdr qt svn
       - name: Configure SDRangel
         run: |
           mkdir build && cd build