sccache v0.2.13 fails when -Z unstable-options are specified

[SomeoneToIgnore]

Minimal example:

• use nightly rustc: docker run -it --rm rustlang/rust:nightly bash
• install sccache: rustup toolchain install 1.47.0 && cargo +1.47.0 install sccache (I have to use 1.47 due to sccache v0.2.13 does not work when built with rustc 1.48 #887)
• create a dummy project: USER=test-user cargo new --bin test-project && cd test-project
• run cargo clean && cargo clippy -Z unstable-options it works
• run cargo clean && RUSTC_WRAPPER="/usr/local/cargo/bin/sccache" cargo clippy -Z unstable-options

Expected: it works too.
Actual: error,

error: failed to execute compile
caused by: Compiler not supported: "error: Unrecognized option: \'E\'\n\n"
error: could not compile `test-project`

[SomeoneToIgnore]

Seems to work on current master

[glandium]

Closing per #889 (comment)