fix `x clean` for cross-compiled artifacts ```toml build = "x86_64-unknown-linux-gnu" host = ["arm-unknown-linux-gnueabihf"] target = ["arm-unknown-linux-gnueabihf"] ``` On `x86_64-unknown-linux-gnu`, after cross-compiling with the sample configuration above, artifacts under `build/x86_64-unknown-linux-gnu` never gets cleaned with `x clean`. This PR fixes that.