OP_SESSION environment variable not found

I am unable to perform the migration as `OP_SESSION` variable is always empty

```bash
$ eval $(op signin)
$ secrethub migrate plan ###
Encountered an error: OP_SESSION environment variable not found, run `eval $(op signin)` to set one
```
cli versions:
```bash
$ secrethub -v
secrethub version 0.44.0, build d29a942
$ op -v
2.7.1
```
