Remove references to lockfileFilename

This field will be removed from yarn in version 4: https://github.com/yarnpkg/berry/pull/5604

With this change, the plugin works with the current canary release, v4.0.0-rc.53.

src/generate.ts

@@ -93,7 +93,7 @@ export default async (
     "./" +
     ppath.relative(
       ppath.dirname(nixExprPath),
-      ppath.resolve(cwd, configuration.get(`lockfileFilename`))
+      ppath.resolve(cwd, 'yarn.lock' as PortablePath)
     );
 
   // Build a list of cache entries so Nix can fetch them.