Both CI for "rustc repo" and "verus repo" runs on a CMU server. It is connected as a "self-hosted runner" on github action.
Adding a self-hosted runner is possible on the settings tab on github repository. "Settings->Actions->Runners".

"rust repo" is for our custum rust compilation. "verus repo" runs CI test using the lastest binary of our custum rust.
    
For rust repo, CI is just for building and uploading binary for now. It is expected to be done within an hour.
Note that we are currently skipping the CI tests that offical rust lang repo is doing. It would be nice to do some test that officla rust-lang CI does.
   
rust-lang compiler community uses "x.py" for building rust.
x.py will use "config.toml" for its configuration
if you want to update our rust compile configuration, 
make changes at config.toml.verify, not config.toml
    
In the compile-rust.yml file, it is assumed that  
prefix = "install" in config.toml.verify 
search "General install configuration options" and "[install]" in config.toml file for detailed information on this.
   
For Verus repo, it first graps the latest binary from above repo. 
However, since two CI is running on the same machine, it can be simply done by "ln -s" or "cp -r".

Note that Z3 binary path("VERUS_Z3_PATH") and Singular binary path("VERUS_SINGULAR_PATH") are needed to run tests.
If something breaks, it is worth checking .cargo/config.toml file.

To change the Z3 version in CI, change it in `.github/workflows/git-z3.sh`, since `ci.yml` will download the Z3 binary using that script.