Remove temporary `GOPATH` creation for codegen and openapigen.

[twoGiants]

Summary

Running make generated takes locally 15-20min, under the hood the setup-temporary-gopath.sh is called which uses the legacy GOPATH configuration, creates a temporary directory and downloads all the dependencies every time once for codegen and once for openaipgen.

You appear to be running from outside of GOPATH.
This script will create a temporary GOPATH at /tmp/tmp.6VNP0PcJTs/.gopath for code generation.
Moving to /tmp/tmp.6VNP0PcJTs/.gopath/src/github.com/tektoncd/pipeline

Running make generated without setup-temporary-gopath.sh takes under 2min.

Tasks

• Remove setup-temporary-gopath.sh from update-codegen.sh and update-openapigen.sh.
• Validate build, tests and CI remain running.

[twoGiants]

/kind cleanup

/assign