misc/wasm: use temporary directory provided by Node.js

os.TempDir() did not return a proper directory on Windows with js/wasm,
because js/wasm only uses the Unix variant of TempDir.

This commit passes the temporary directory provided by Node.js to the
Go runtime by adding it as a default value for the TMPDIR environment
variable. It makes TempDir compatible with all platforms.

Fixes #27306.

Change-Id: I8b17e44cfb2ca41939ab2a4f918698fe330cb8bc
Reviewed-on: https://go-review.googlesource.com/c/150437


Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>