Generated code may go to $GOPATH/src instead of in repo

[stmcginnis]

/kind bug

What steps did you take and what happened:

Found in kubernetes-sigs/cluster-api#4185

We use conversion-gen to generate some of the code. This command has an optional argument of --output-base that controls where the generated files are placed.

The default for this arg can vary. If GOPATH is set, then it will place files under $GOPATH/src. But if not set, it defaults to ./, which is what we want to have the generated files within the repo.

The dependency on an environment variable is non-obvious and can cause stale files to be used if the user doesn't know about it. There also appears to be a difference whether running with the repo cloned within $GOPATH/src or elsehwere.

What did you expect to happen:

Files generated/over-written within the repo. In environment with GOPATH set, files were generated under $GOPATH/src/api.