Nightly 2021-07-03

web: Fix build without wasm-opt on Windows

Since #3111, `wasm-opt` became mandatory on GitHub Actions. But the
check whether we run on GitHub Actions is Bash-specific, and fails
on Windows. That means builds on Windows without `wasm-opt` always
fail.

As a workaround, use the trick from https://gist.github.com/prail/24acc95908e581722c0e9df5795180f6
to run the check only on Unix, and skip it on Windows:

```
echo ; [ \"$GITHUB_ACTIONS\" != true ] # > nul
```

On Unix, `echo` does nothing, the check runs, and the rest is
considered a comment.
On Windows, `echo` prints everything until the `> nul`, which
suppresses the print by redirection, making the whole statement
effectively a no-op.