agent: rename cargo config file

The .cargo/config file is obsolete. Rust is using config.toml instead,
since v1.39
(see https://doc.rust-lang.org/cargo/reference/config.html)

This is not a problem in general as Cargo will understand both, the first
taking precedence over the second.
But our prefetcher mecanism for hermetic builds doesn't, and this prevents
the build to run.

This has been logged as an error on Hermeto:
hermetoproject/hermeto#946

Now the file itself has already been renamed upstream:
kata-containers@0d8242a

So sooner or later, we should get rid of this commit.

Signed-off-by: Julien Ropé <[email protected]>

Author: littlejawa