🐛 fix: staging image build failed due to toolchain directive

[richardcase]

What type of PR is this?

/kind bug

What this PR does / why we need it:

When doing a recent release of CAPA we encountered an issue with the build of the staging image:

/workspace/hack/tools/go.mod:5: unknown directive: toolchain
go: errors parsing go.mod:

(source)

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:

We'll need to update the cloud build config on main as well.

Checklist:

• squashed commits
• includes documentation
• includes emojis
• adds unit tests
• adds or updates e2e tests

Release note:

Remove toolchain directive and updated build image

[damdo]

/lgtm

Will need title to be conforming

[richardcase]

/approve

[k8s-ci-robot]

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