The 'doc' builder is sporadically failing

[alexcrichton]

It occasionally succeeds, but it's frequently failing with this message:

/home/rustbuild/src/rust-buildbot/slave/doc/build/src/driver/driver.rs:29:0: 29:32 error: multiple matching crates for `rustc`
/home/rustbuild/src/rust-buildbot/slave/doc/build/src/driver/driver.rs:29 extern mod this(name = "rustc");
                                                                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
note: candidates:
note: path: /home/rustbuild/src/rust-buildbot/slave/doc/build/obj/x86_64-unknown-linux-gnu/stage0/lib/rustc/x86_64-unknown-linux-gnu/lib/librustc-d3cb8c2ccd84a7a7-0.7-pre.so
note: meta: name = "rustc"
note: meta: vers = "0.7-pre"
note: meta: uuid = "0ce89b41-2f92-459e-bbc1-8f5fe32f16cf"
note: meta: url = "https://github.com/mozilla/rust/tree/master/src/rustc"
note: path: /home/rustbuild/src/rust-buildbot/slave/doc/build/obj/x86_64-unknown-linux-gnu/stage0/lib/rustc/x86_64-unknown-linux-gnu/lib/librustc-961c3123ba57818-0.7-pre.so
note: meta: name = "rustc"
note: meta: vers = "0.7-pre"
note: meta: uuid = "0ce89b41-2f92-459e-bbc1-8f5fe32f16cf"
note: meta: url = "https://github.com/mozilla/rust/tree/master/src/rustc"
error: aborting due to previous error
make: *** [x86_64-unknown-linux-gnu/stage0/lib/rustc/x86_64-unknown-linux-gnu/bin/rustc] Error 101

@graydon, could a make clean be inserted as a step for this builder? When looking at http://buildbot.rust-lang.org/grid?width=20&branch=master, it looks like this may actually be just one workspace needing a clean.

[alexcrichton]

Probably caused by or related to #2320

[thestinger]

cc @graydon

[alexcrichton]

It appears to be consistently succeeding now, I believe this was fixed in 033ac54