Add Exportloopref linter and fix errors

[Hoega]

What type of PR is this?

/kind bug

What this PR does / why we need it

Avoid the "loop variable trap" as described in exportloopref

Which issue(s) this PR fixes

Fixes #541

Special notes for your reviewer

Fix the following errors returned by golangci-lint run -E exportloopref :

apiserver/pkg/dataset/dataset.go:57:27: exporting a pointer for the loop variable cond (exportloopref)
                                ds.UpdateTimestamp = &cond.LastTransitionTime.Time
                                                      ^
apiserver/pkg/ray/raycluster.go:47:20: exporting a pointer for the loop variable cluster (exportloopref)
                        HeadAddress:   &cluster.HeadAddress,
                                        ^
apiserver/pkg/ray/raycluster.go:48:20: exporting a pointer for the loop variable cluster (exportloopref)
                        DashboardHost: &cluster.DashboardHost,
                                        ^
apiserver/pkg/ray/raycluster.go:49:20: exporting a pointer for the loop variable cluster (exportloopref)
                        PythonVersion: &cluster.PythonVersion,
                                        ^
apiserver/pkg/versioneddataset/versioned_dataset.go:87:28: exporting a pointer for the loop variable cond (exportloopref)
                                vds.UpdateTimestamp = &cond.LastTransitionTime.Time

[Abirdcfly]

we can use rebase merge for this pull request, then we will get only one commit in git log.