[Go SDK] error on unmarshalling nested secure_vault

https://github.com/ytsaurus/ytsaurus/blob/770aca260c9f3e6b017f9c3c11c25b5b168e4966/yt/go/mapreduce/io.go#L31-L39

This code assumes that secure vault values are always string, which is not true for docker_auth secret (https://ytsaurus.tech/docs/en/user-guide/data-processing/layers/layer-paths#docker_auth). It leads to the following error:
```
job: corrupted secure vault:
go.ytsaurus.tech/yt/go/mapreduce.(*jobContext).initEnv
/go/pkg/mod/go.ytsaurus.tech/yt/go@v0.0.28/mapreduce/io.go:36
- yson: cannot unmarshal map into value of type []uint8
```