add option to force usage of fallback_auth_store

[0xbe7a]

When running within a CI, it is sometimes easier not to rely on the native authentication toolchain, but to use a simple credentials file instead (see prefix-dev/rattler-build#334). This PR adds an option to force the use of this fallback file and disable the use of the system toolchain.

[wolfv]

I think this is "OK" for me. Not a super-fan of the option :)

I am wondering if we should rather create another authentication storage that can be supplied instead of the current one. The new one would rely only on the fallback storage and uses plain JSON as database, and then we would have a Rust trait for an authentication storage?

[0xbe7a]

Yeah, that sounds a bit more elegant.

[0xbe7a]

I still need to test this with rattler-build

[wolfv]

Nice work! Looks good to me. If we can get rid of anyhow it's perfect :)

[0xbe7a]

I adapted and tested the rattler-build PR with our real-world example. Works great! I also changed the file location of the .json file, as the format changed slightly and we now store valid JSON.

[0xbe7a]

Would you prefer to return a Box<dyn Error + Send + Sync>instead of an anyhow Result? Since I wanted to leave the option for users to bring your their own backend, you can't really type the error type.

[wolfv]

right. let's try to ask @baszalmstra what he thinks is best.

[baszalmstra]

Thanks for this PR, almost looks good to me. I left some small comments.

And I would just use anyhow Result.