[infra] Fix index.d.ts copy in Wasm Docker build

Change-Id: I0e6a74c02186c97e8f1ca676c3593c4729637654
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/374216
Reviewed-by: Kevin Lubick <kjlubick@google.com>
1 file changed