fix: failed to parge a map in meta: cannot convert to object: map[interface {}]interface {}

[suzuki-shunsuke]

How to reproduce

.buildflow.yaml

---
phases:
- name: main
  tasks:
  - name: hello
    meta:
      foo:
        bar: zoo
    command:
      command: echo hello

$ buildflow run
==============
= Phase: main =
==============
10:03:39UTC | hello | + /bin/sh -c echo hello
10:03:39UTC | hello |
10:03:39UTC | hello | hello
10:03:39UTC | hello |

================
= Phase Result: main =
================
status: failed
error: cannot convert to object: map[interface {}]interface {}
task: hello
status: succeeded
exit code: 0
start time: 2020-10-15T10:03:39Z
end time: 2020-10-15T10:03:39Z
duration: 4.232052ms
+ /bin/sh -c echo hello
hello

FATA[0000] cannot convert to object: map[interface {}]interface {}

[suzuki-shunsuke]

related: https://github.com/suzuki-shunsuke/tengo-tester/issues/6

[suzuki-shunsuke]

https://github.com/suzuki-shunsuke/go-convmap

[suzuki-shunsuke]

80