Set GOMODCACHE to avoid re-download toolchain

[huww98]

If we don't set this, k/k build script will set GOMODCACHE different from default, and re-download the toolchain (or fail to build for Kubernetes <= 1.29).

/release-note-none

[k8s-ci-robot]

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[huww98]

/release-note-none

[avorima]

@huww98 i see GOCACHE is also being set in the k/k scripts. should that also be overwritten here?

[xing-yang]

/ok-to-test

[xing-yang]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: huww98, xing-yang

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