can't add a package to the SDK that doesn't build ;)

https://uberchromegw.corp.google.com/i/chromiumos.tryserver/builders/chromiumos-sdk/builds/2630/steps/SDKBuildToolchains/logs/stdio

>>> Emerging (1 of 1) dev-util/cargo-0.20.0::portage-stable
error: variable does not need to be mutable
    --> src/cargo/core/resolver/mod.rs:1095:17
     |
1095 |             let mut set = self.resolve_features.entry(pkgid.clone())
     |                 ^^^^^^^
     |
note: lint level defined here
    --> src/cargo/lib.rs:1:9
     |
1    | #![deny(unused)]
     |         ^^^^^^
     = note: #[deny(unused_mut)] implied by #[deny(unused)]

error: variable does not need to be mutable
  --> src/cargo/util/read2.rs:15:18
   |
15 |                  mut data: &mut FnMut(bool, &mut Vec<u8>, bool)) -> io::Result<()> {
   |                  ^^^^^^^^

error: aborting due to 2 previous errors

error: Could not compile `cargo`.