The current cargo2android.py doesn't work well w/ `--cargo_bin`. It would use the full path rustc(/<HOME>/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc) instead of basename(`rustc`). The `RUSTC_PAT` and `RUSTC_VV_CMD_ARGS` regex doesn't match the full path.
This CL uses `(.*\/)?` to match the path if it exists.
Change-Id: If5e4060410c1cd45ad6c9612ebe856552aa14d08