Fix Ballista rust.yml github workflow

[RaphaelMarinier]

Fix the failing docker builder in the github workflow.

Make clippy happy:

• Remove unused ExecutorsResponse.
• Allow an 'assigning_clone'.

[andygrove]

@RaphaelMarinier looks like you need to add rust-toolchain to rat exclude list (somewhere under dev, IIRC)

NOT APPROVED: rust-toolchain.toml (./rust-toolchain.toml): false

[RaphaelMarinier]

I added the missing license header to rust-toolchain.toml to make rat happy.

[andygrove]

Thanks @RaphaelMarinier