This commit fixes two things:
- it ensures the code generation works no matter the location of the
project directory
- as a side effect; fixes the `hack/verify-codegen.sh` ran during CI
The previous script (or more specific: the `code-generator` library)
made the assumption during execution that the project was placed
inside `$GOPATH/src` and made the modifications there.
The idea of Go Modules is however that a project and/or package can
be placed anywhere, and this is also what the CI did, resulting in a
comparison of two identical `cp -r` copied directories. Giving a
false green light on every CI run.
To work around this limitation in `code-generator`: create a
temporary directory, use this as an output base and copy
everything back once generated.
