In v2.MetadataTest, `envVar` property is actually `env`

[zephinzer]

According to the patterns for converting the listed properties into YAML properties from the README.md, it seems like the EnvVars should correspond to a property named envVars but it is actually env (checking from the code at https://github.com/GoogleContainerTools/container-structure-test/blob/master/pkg/types/v2/metadata.go#L26). Should it be renamed to envVars or should the docs be changed to env?

[donmccasland]

Nice catch. I'll investigate.