Make the pure build work

nix/crate-overrides.nix

@@ -20,26 +20,23 @@ defaultCrateOverrides
   rav1e = attrs: { env.CARGO_ENCODED_RUSTFLAGS = "-C target-feature=-crt-static"; };
 
   ffmpeg-sys-next = attrs: {
-    nativeBuildInputs = (attrs.nativeBuildInputs or []) ++ [
+    nativeBuildInputs = [
       pkg-config
     ];
-    buildInputs = (attrs.buildInputs or []) ++ [
-      llvmPackages.libclang
-      ffmpeg.dev
-      gcc.cc
-      gcc-unwrapped
-      stdenv
+    buildInputs = [
+      rustPlatform.bindgenHook
+      ffmpeg
     ];
-    env = (attrs.env or {}) // {
-      LIBCLANG_PATH = "${llvmPackages.libclang.lib}/lib";
-      CPATH = "${gcc-unwrapped}/lib/gcc/${stdenv.hostPlatform.config}/${gcc-unwrapped.version}/include";
-      BINDGEN_EXTRA_CLANG_ARGS = builtins.concatStringsSep " " [
-        "-I${gcc.libc.dev}/include"
-        "-I${gcc}/lib/gcc/x86_64-unknown-linux-gnu/${gcc.version}/include"
-        "-I${llvmPackages.libclang.lib}/lib/clang/${llvmPackages.libclang.version}/include"
-      ];
-      PKG_CONFIG_PATH = "${ffmpeg.dev}/lib/pkgconfig";
   };
+
+  ffmpeg-next = attrs: {
+    # Somehow the variables that are passed are mangled, so they are not
+    # correctly passed to the ffmpeg-next build script. Worth investigating
+    # more since it's probably a bug in crate2nix or buildRustCrate.
+    postPatch = ''
+      substituteInPlace build.rs \
+        --replace-fail "DEP_FFMPEG_" "DEP_FFMPEG_SYS_NEXT_"
+    '';
   };
 
   grpc-metadata = attrs: {