`cog predict` error when unmarshaling run item from `cog.yaml`

[mattt]

Related to #1010

$ cog predict r8.im/daanelson/stable-diffusion-speed-lab@sha256:db21e45d3f7023abc2a46ee38a23973f6dce16bb082a930b0c49861f96d1e5bf -i prompt=guitar
ⅹ Failed to parse config from r8.im/daanelson/stable-diffusion-speed-lab@sha256:db21e45d3f7023abc2a46ee38a23973f6dce16bb082a930b0c49861f96d1e5bf: json: cannot unmarshal string into Go struct field Build.build.run of type config.RunItem

RunItem has an implementation for UnmarshalYAML but is missing a corresponding UnmarshalJSON.