Add support for nix store remote copying

[z8v]

This is a requirement in order to make Bazel remote execution work. Without it it's not guaranteed that any necessary nix paths will be available during the build.

Any local derivation will be copied over to a specified remote nix server which will also as a Bazel executor. This way any nix paths required during the build, will be available.

The user will have to provide the BAZEL_NIX_REMOTE environment variable. This should be the name of any entry on the SSH_CONFIG file where all the authentication details are provided.

e.g

$ export BAZEL_NIX_REMOTE=nix-server

$ cat ~/.ssh/config
Host nix-server
  Hostname 1.2.3.4
  IdentityFile ~/.ssh/nix-server
  Port 2222
  User nix-user

This was done in order to simplify the processes of authentication and keep the number of configuration options to a minimum.

[z8v]

@benradf I believe it would make sense to include in this PR a docker/vagrant setup to test the remote build on the CI?

[benradf]

That sounds like a great idea actually, though I'm not entirely sure how to go about it.

What were you thinking? Build a docker image as part of CI, run it, and test that it can be used as a remote executor?

[z8v]

That sounds like a great idea actually, though I'm not entirely sure how to go about it.

What were you thinking? Build a docker image as part of CI, run it, and test that it can be used as a remote executor?

I was thinking to have a container that will play the role of the nix server. We could run this on the CI so we can test the code path that is enabled when we specify a remote nix server.

[benradf]

Maybe add something like ${{ matrix.withNixRemote && '- NixRemote' || '' }} to the end of the job name in workflow.yaml to distinguish jobs on the withNixRemote dimension.

Otherwise, looks good 👍

[z8v]

Maybe add something like ${{ matrix.withNixRemote && '- NixRemote' || '' }} to the end of the job name in workflow.yaml to distinguish jobs on the withNixRemote dimension.

Otherwise, looks good 👍

Updated the title. We'll need to adjust the required checks on Github with the new name.