Source GOPATH from go env in compile-release-tool script

[embik]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

This is a very minor chore I stumbled over while working on #3559 - when GOPATH isn't set in your environment (and these days, it's not necessary anymore to have it set), make release-tools fails with

./compile-release-tools: line 35: GOPATH: unbound variable

This sources the variable from go env, which prints either the set environment variable or the implied default.

Feel free to close this if it's not really something you want to merge.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?

NONE

[k8s-ci-robot]

Hi @embik. Thanks for your PR.

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[cpanato]

/ok-to-test

[cpanato]

thanks

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cpanato, embik

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