ignore rustwide from Cargo's workspace

[pietroalbini]

No description provided.

[jyn514]

For posterity: this is fallout from #1000 and fixes the following error when building locally:

2020/09/01 14:26:59 [INFO] rustwide::cmd: running `Command { std: "/home/imperio/rust/docs.rs/.workspace/cargo-home/bin/cargo" "+nightly" "read-manifest" "--manifest-path" "Cargo.toml", kill_on_drop: false }`
Error: Building documentation failed

Caused by:
    invalid Cargo.toml syntax