Pin versions for dependencies in Makefile

[nikhita]

The indirect dependencies in go.mod, specifically golang.org/x/tools and github.com/kisielk/errcheck, are not really used anywhere in the code. go mod tidy also removes these lines. They are only used in the Makefile using go get.

Currently, no version is specified while using go get. This means that go will try to fetch the latest version and this version can deviate from the version in go.mod. To avoid this, the PR updates the Makefile to explicitly go get the versions specified in go.mod.

Additionally, this PR also updates the versions to match the versions in v1.3.1 i.e. avoids the version bumps made in #717:

• github.com/kisielk/errcheck to v1.2.0
• golang.org/x/tools to release-branch.go1.15

Context - bumping gogo/protobuf from v1.3.1 to v1.3.2 in kubernetes requires bumping x/tools as well, which we'd like to avoid in k8s (xref kubernetes/client-go#927)

cc @awalterschulze

[nikhita]

fyi @dims @palnabarun

[awalterschulze]

I guess because it is used in CI and I am not familiar with godeps, it is after my time in go land.
Also see #691
Unfortunately we do not have personal capacity to keep on maintaining this project.