fix: escape unicode characters in `get` step

[braunsonm]

• Only replaces on the concourse metadata, not the output file
• Without this, concourse GRPC will fail stating that the string contains unknown UTF-8 even if the body is valid UTF-8 according to go.

I'm happy to explore other options, however go's own utf8.ValidString considers the body from knative/operator as valid UTF-8 whereas Concourse does not. Since this only affects the Concourse metadata on the get step I think this is a reasonable compromise.

Fixes #128