Fix CERTIFICATE_VERIFY_FAILED errors on macOS

[voronin-ilya]

Fixes #1102

[Acly]

This feels somehow even more brittle than the previous solution. At least I couldn't (quickly) find any information that confirms the cert file will always exist in this location (depending on macOS version and what packages are installed)...?

If the issue is that the environment variable typically isn't set at all, maybe it would be better to only set it if it doesn't exist. That way it would still be possible to set a custom value outside at least, without it being overwritten.

[voronin-ilya]

I am unable to confirm that this file is present on every macOS version (although some Googling suggests it most likely is). Which solution do you prefer: setting SSL_CERT_FILE only if it’s not already set, or adding certifi as a submodule and including it as done with the websockets library? The current solution isn’t really working since certifi is not bundled with the Pykrita environment.

[Acly]

I think adding certifi as submodule is overkill.

What might work is calling python -m certifi as a subprocess, so it uses system python instead of Krita's embedded. But this would require python to be installed on the system, which is probably not always the case either.

I generally also don't mind the fixed path, I just think the env shouldn't be overwritten if it was set already, to still allow it being set outside if the hard-coded path doesn't work. Or do that as a fallback.