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>

misc/wasm/wasm_exec.js

@@ -442,7 +442,7 @@
 
 		const go = new Go();
 		go.argv = process.argv.slice(2);
-		go.env = process.env;
+		go.env = Object.assign({ TMPDIR: require("os").tmpdir() }, process.env);
 		go.exit = process.exit;
 		WebAssembly.instantiate(fs.readFileSync(process.argv[2]), go.importObject).then((result) => {
 			process.on("exit", (code) => { // Node.js exits if no callback is pending