Making small changes requires a 200 megabyte download from crates.io each time

When I try running
`python3 infra/helper.py build_fuzzers --sanitizer address rust-brotli /path/to/rust-brotli`

it executes:
```
+ cargo fuzz build
Updating crates.io index
Fetch [==================> ] 76.85%, (122797/187670) resolving....
```
This puts undue burden on my network and the rust infrastructure to try to reproduce a problem with changes.
Improving the compile/eval/debug cycle could dramatically improve author engagement.