fix to pass workcache::test on android

[yichoi]

To enable test on android bot #9120

test workcache::test ... task '<unnamed>' failed at 'Unhandled condition: io_error: rt::io::IoError{kind: OtherIoEr
ror, desc: "no such file or directory", detail: None}', /home/yichoi/rust_latest/src/libstd/condition.rs:131
task 'workcache::test' failed at 'receiving on closed channel', /home/yichoi/rust_latest/src/libstd/rt/comm.rs:198
FAILED

[emberian]

workcache is gone