Fix: env() does not always return platform

[jbransen]

In attempts at running TensorFlow in an AudioWorklet (see #4704 ), I ran into the issue where env().platform is undefined. This is the pragmatic fix, since the fallback is supposed to be removed in the future. (and to actually run TFJS in an AudioWorklet, I needed some more patches to the WASM backend loading, but that's unrelated to this PR).

[pyu10055]

/gcbrun

[oneil512]

Any progress on this?