ffmpeg: needs gcc

https://hydra.nixos.org/build/75640051/nixlog/2

pkgs/top-level/all-packages.nix

···
 
   ffmpeg_0_10 = callPackage ../development/libraries/ffmpeg/0.10.nix {
     inherit (darwin.apple_sdk.frameworks) Cocoa;
+    stdenv = gccStdenv;
   };
   ffmpeg_1_2 = callPackage ../development/libraries/ffmpeg/1.2.nix {
     inherit (darwin.apple_sdk.frameworks) Cocoa;
+    stdenv = gccStdenv;
   };
   ffmpeg_2_8 = callPackage ../development/libraries/ffmpeg/2.8.nix {
     inherit (darwin.apple_sdk.frameworks) Cocoa;
+    stdenv = gccStdenv;
   };
   ffmpeg_3_4 = callPackage ../development/libraries/ffmpeg/3.4.nix {
     inherit (darwin.apple_sdk.frameworks) Cocoa CoreMedia;
···
   };
   ffmpeg_4 = callPackage ../development/libraries/ffmpeg/4.nix {
     inherit (darwin.apple_sdk.frameworks) Cocoa CoreMedia;
+    stdenv = gccStdenv;
   };
 
   # Aliases