Use proper path for GOPATH/pkg cache

[xmudrii]

This should be the proper path based on proxy.golang.org.

e.g.

$ wget 'https://proxy.golang.org/github.com/kcp-dev/apimachinery/v2/@v/v2.0.0.mod
# works

$  wget 'https://proxy.golang.org/github.com/kcp-dev/apimachinery/@v/v2/v2.0.0.mod
# fails

let's hope this fixes the issue that we're seeing right now.

/assign @xrstf @embik

[embik]

/approve

[kcp-ci-bot]

LGTM label has been added.

Git tree hash: 61000f3247fd840d4be32827ce2bc0f5303e0a8c

[kcp-ci-bot]

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