Force environment variables when running on iOS

[buggmagnet]

This PR tricks rust into believing environment variables have been set properly when running on iOS.

With this, we can then add the --features flag when building crates for iOS to allow overriding the API when running in a staging environment for example.

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This change is 

[linear]

IOS-541 Simulate overriden environment variable when running on iOS

[dlon]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @buggmagnet)

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mullvad-api/src/ffi/mod.rs line 71 at r1 (raw file):

        })?;

        std::env::set_var(crate::env::API_HOST_VAR, &api_hostname);

The arguments to new will be ignored for any clients but the first one, since the environment variables are only used to lazily initialize a global static (API). If it's not worth fixing, can you add a doc comment pointing this out?

[dlon]

Reviewed 2 of 2 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved

[github-actions]

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