1.30.0-beta cannot bootstrap itself

[Keruspe]

Current beta (14f51b0) cannot bootstrap itself.
The cargo version shipped with it appends --color=always when the output terminal supports it, and src/bootstrap/compile.rs sets RUSTC_COLOR=1 leading to src/boostrap/bin/rustc.rs also appending --color=always.
This results in commands like:

rustc command: "LD_LIBRARY_PATH"="/usr/host/lib:/var/tmp/paludis/build/dev-lang-rust-1.30.0-scm/work/rust-1.30.0-scm/build/x86_64-unknown-linux-gnu/stage0-std/release/deps:/usr/x86_64-pc-linux-gnu/lib" "/usr/host/bin/rustc-beta" "--crate-name" "cc" "/var/cache/paludis/distfiles/cargo-home/registry/src/github.heygears.com-1ecc6299db9ec823/cc-1.0.22/src/lib.rs" "--color" "always" "--crate-type" "lib" "--emit=dep-info,link" "-C" "opt-level=2" "-C" "metadata=31edc2328efb244d-rustc" "-C" "extra-filename=-31edc2328efb244d" "--out-dir" "/var/tmp/paludis/build/dev-lang-rust-1.30.0-scm/work/rust-1.30.0-scm/build/x86_64-unknown-linux-gnu/stage0-std/release/deps" "-L" "dependency=/var/tmp/paludis/build/dev-lang-rust-1.30.0-scm/work/rust-1.30.0-scm/build/x86_64-unknown-linux-gnu/stage0-std/release/deps" "--cap-lints" "allow" "--cfg" "stage1" "-Clinker=x86_64-pc-linux-gnu-cc" "-C" "target-feature=-crt-static" "--color=always"

which in turn leads to

error: Option 'color' given more than once
error: Could not compile `cc`