Makefile: avoid redundant work upon make restart

[thockin]

Only build and run go2make on the first pass. If that generates a new
GO_PKGDEPS_FILE, make will restart the whole process and set MAKE_RESTARTS to
a numeric value.

We can use this to avoid re-running go2make, which saves a few seconds
each build.

/kind cleanup

NONE

[k8s-ci-robot]

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[dims]

/approve
/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dims, thockin

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[thockin]

How to kick easyCLA?

[dims]

/easycla