[release-1.7] 🌱 Makefile: refactor how govc is built to workaround dependency issues building govc

[chrischdi]

What this PR does / why we need it:

Manual cherry-pick of #1986 to release-1.7

Which issue(s) this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when PR gets merged):
Fixes #

Special notes for your reviewer:

Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

[sbueringer]

/lgtm
/retest-required

/assign @randomvariable

[randomvariable]

/approve
/retest-required

[k8s-ci-robot]

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[k8s-ci-robot]

@chrischdi: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-cluster-api-provider-vsphere-verify-gen	9640634	link	false	/test pull-cluster-api-provider-vsphere-verify-gen

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[randomvariable]

/retest-required