feat(runner): add env overrides for safari and ff

[Zekfad]

adds SAFARI_EXECUTABLE and FIREFOX_EXECUTABLE
this allows you to use FF Developer Edition and Safari Technology Preview.
Also there are already MS_EDGE_EXECUTABLE that were missing test.

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[Zekfad]

For anyone with problems running FF: here's the catch:
For some reason you must use either --headless flag, or set MOZ_AUTOMATION env variable to true.
Ref: https://github.com/mozilla/gecko-dev/blob/6755226f52a02eac8b18f25a6f522e9c0814b628/browser/app/winlauncher/LauncherProcessWin.cpp#L165-L167

Update:
Found that you must use --wait-for-browser to make launcher process wait until browser exits.

[natebosch]

Can you add a CHANGELOG entry?

[Zekfad]

@natebosch, added changelog entry to current 1.24.4-wip.