使用 go1.12 mod 引入包报错 exec: "bzr": executable file not found in %PATH

``` go: finding github.com/lexkong/log latest go: finding github.com/smallnest/rpcx latest go: finding github.com/kataras/iris/context latest go: finding github.com/kataras/iris/core/errors latest go: finding github.com/rcrowley/go-metrics latest go: finding github.com/kataras/iris/core latest go: finding github.com/golang/lint v0.0.0-20180702182130-06c8688daad7 go: finding golang.org/x/lint v0.0.0-20180702182130-06c8688daad7 go: labix.org/v2/mgo@v0.0.0-20140701140051-000000000287: bzr branch --use-existing-dir https://launchpad.net/mgo/v2 . in D:\go\pkg\mod\cache\vcs\ca61c737a32b1e09a0919e1 5375f9c2b6aa09860cc097f1333b3c3d29e040ea8: exec: "bzr": executable file not found in %PATH% ``` 这是说环境变量少了啥????