env/js-wasm: update to Node.js v13.0

I'm hoping the V8 bump gets us past some tests that were too slow
before.

Updates golang/go#31282 (slow x/text/unicode/norm)

Change-Id: I80bdded59c0148c4d3f277acc82c5cac7a339666
Reviewed-on: https://go-review.googlesource.com/c/build/+/202641
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

env/js-wasm/Dockerfile

@@ -8,7 +8,7 @@ MAINTAINER golang-dev <golang-dev@googlegroups.com>
 ENV DEBIAN_FRONTEND noninteractive
 
 
-RUN (curl -sL https://deb.nodesource.com/setup_12.x | bash -) && \
+RUN (curl -sL https://deb.nodesource.com/setup_13.x | bash -) && \
     apt-get install -y nodejs && \
     rm -rf /var/lib/apt/lists/*