feat: support private caches

[vetsin]

Fix for #1024 -- enables the usage of the cachix env var.

Note, this does not support the netrc, as that would seemingly be more complicated

$ ~/projects/devenv/result/bin/devenv test
• Overriding .devenv to .devenv.HbfeKKcHpORj
• Building tests ...
• Using Cachix: devenv, private
• Trusting private.cachix.org on first use with the public key private.cachix.org-1:blabla
✔ Building tests in 11.2s.

[vetsin]

@domenkozar fixed the rustfmt, didn't realize i should probably run the tests first :) if you could kindly re-trigger so i could get this downstream

[vetsin]

I see failed tests, but seemingly none are because of my change? They seem flaky? Any input here?