enable widevine for chromium and firefox

packages/widevine-firefox/overlay.nix

@@ -0,0 +1,3 @@
+final: prev: {
+  widevine-firefox = prev.callPackage (builtins.path { path = ./package.nix; }) { };
+}

packages/widevine-firefox/package.nix

@@ -0,0 +1,19 @@
+{
+  stdenv,
+  widevine-cdm,
+}:
+
+stdenv.mkDerivation {
+  name = "widevine-firefox";
+  version = widevine-cdm.version;
+
+  buildCommand = ''
+    mkdir -p $out/gmp-widevinecdm/system-installed
+    ln -s "${widevine-cdm}/share/google/chrome/WidevineCdm/manifest.json" $out/gmp-widevinecdm/system-installed/manifest.json
+    ln -s "${widevine-cdm}/share/google/chrome/WidevineCdm/_platform_specific/linux_arm64/libwidevinecdm.so" $out/gmp-widevinecdm/system-installed/libwidevinecdm.so
+  '';
+
+  meta = widevine-cdm.meta // {
+    platforms = [ "aarch64-linux" ];
+  };
+}