misc/wasm: extend executable compatibility

misc/wasm: extend executable compatibility

The path /bin/bash is not available on all operating systems.
Use /usr/bin/env bash to find the system bash interpreter.

Change-Id: I493e462a8e261b7fbbd3f3c0b1d10e55c5ed783b
Reviewed-on: https://go-review.googlesource.com/c/go/+/463977
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Johan Brandhorst-Satzkorn <johan.brandhorst@gmail.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Evan Phoenix <evan@phx.io>