[Bug rust/119333] [15 regression] Rust bootstrap fails with cargo trying to download polonius crates

[sjames at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119333

--- Comment #2 from Sam James <sjames at gcc dot gnu.org> ---
.cargo/ at top-level with:

.cargo/config.toml:
```
[net]
offline = true
```

and CARGO_HOME="${srcdir}/.cargo" or something like that would work.