[ignite] Fix multiple main packages found

When downloading the `vendor/` directory with:
```
$ go mod vendor
```
and then e.g. run `ignite chain init` this error happens:

* specify the path to your chain's main package in your
  config.yml>build.main: multiple main packages found

Signed-off-by: Julian Strobl <jmastr@mailbox.org>

config.yml

@@ -1,5 +1,7 @@
 ---
 version: 1
+build:
+  main: cmd/planetmint-god
 accounts:
   - name: alice
     coins: