allow for project specific rust file server mirrors

[dirkboye]

We mirror all external dependencies to our local network so that we do not need any internet access for building our software.
Using environment variable STATIC_RUST_URL is good for our CI but it's cumbersome for developers who need
to have the environment variable set as well.

A better solution can be seen in go_rules method go_download_sdk ( see https://github.com/bazelbuild/rules_go/blob/master/go/toolchains.rst#go-download-sdk ) where you can set the mirror urls.

I'd like to propose to add something similar to rust_toolchain_repository which would make it possible to
have project-specific rust mirrors.

[dirkboye]

PR was merged!

[UebelAndre]

@dirkboye Oh, one other thing I just came accross this morning. You might have been able to add a .bazelrc file to your projects and specify a repo_env. Would that have also worked for you?

[dirkboye]

@UebelAndre unfortunately that doesn't work so well and increases build times for our CI as we use bazel query in our scripts.

bazel query does not support --repo_env or --action_env

running bazel build with repo_env will then result in

INFO: Build option --repo_env has changed, discarding analysis cache.

[UebelAndre]

Oh wow, really good to know. That seems like a bug on Bazel's side though 😞 I otherwise don't understand why --repo_env shouldn't be supported by all commands

[dirkboye]

@UebelAndre here's an open issue on the bazel side for that:
bazelbuild/bazel#11943

[UebelAndre]

@dirkboye Oh wow, you're a hero! Thank you 🙏 😄