Update k8s-ci-builder to 1.15.8

[cpanato]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Since the golang updates in k/k are merged we now can bump the internal image versions, too.

Part of #1895

/assign @saschagrunert @hasheddan @puerco @hasheddan @ameukam

Which issue(s) this PR fixes:

None

Special notes for your reviewer:

Does this PR introduce a user-facing change?

Update k8s-ci-builder to 1.15.8

[cpanato]

this

release/images/releng/k8s-ci-builder/variants.yaml

Lines 8 to 25 in 027eec4

	'1.20':
	CONFIG: '1.20'
	GO_VERSION: '1.15.5'
	BAZEL_VERSION: '3.4.1'
	OLD_BAZEL_VERSION: '2.2.0'
	SKOPEO_VERSION: 'v1.2.0'
	'1.19':
	CONFIG: '1.19'
	GO_VERSION: '1.15.3'
	BAZEL_VERSION: '2.2.0'
	OLD_BAZEL_VERSION: '0.23.2'
	SKOPEO_VERSION: 'v1.2.0'
	'1.18':
	CONFIG: '1.18'
	GO_VERSION: '1.13.15'
	BAZEL_VERSION: '2.2.0'
	OLD_BAZEL_VERSION: '0.23.2'
	SKOPEO_VERSION: 'v1.2.0'

we update after the cherry-picks correct?

[saschagrunert]

/lgtm

[k8s-ci-robot]

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