fix: Override other implementations on custom feature

[VisualEhrmanntraut]

Fixes #500

[newpavlov]

We intentionally do not allow to overwrite "default" entropy sources with custom ones. This may change with #498, but your current approach certainly is a no-go.

[VisualEhrmanntraut]

@newpavlov How about I instead make a feature named prefer-custom or use-custom which will make it prefer the custom implementation to the built-in ones?

[newpavlov]

Renaming the feature does not solve the root security issue with overriding default entropy sources. As I wrote in the issue, at the moment, your best option is to simply patch the getrandom crate with your custom version or introducing a proper custom target for your experiments.