navigator.userAgent is unreliable in determining if the browser is webkit on linux

[MarcoPolo]

Seeing failure in CI and locally on a linux box that should have been fixed by #313. The problem is that the user agent reported is "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/605.1.15 (KHTML, like Gecko) Version/16.4 Safari/605.1.15", which doesn't let us know if this is linux or mac.

Seeing this issue on adding webkit to js-libp2p libp2p/js-libp2p#1627 on this run.

Not sure why the CI here isn't hitting it. Maybe the browser was updated? I'll run the CI again to check.

[MarcoPolo]

The browser was probably updated. See new failure in this repo: https://github.com/libp2p/js-libp2p-crypto/actions/runs/4608311179/jobs/8670470410#step:5:1057