Add dedicated make target for helm

[klocke-io]

What would you like to be added:
https://github.com/gardener/dashboard/pull/2420/files#r2094999964

move the install step for helm in the ./hack/verify script into ints own make target
Why is this needed:

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[gardener-ci-robot]

The Gardener project currently lacks enough active contributors to adequately respond to all issues.
This bot triages issues according to the following rules:

• After 90d of inactivity, lifecycle/stale is applied
• After 60d of inactivity since lifecycle/stale was applied, lifecycle/rotten is applied
• After 60d of inactivity since lifecycle/rotten was applied, the issue is closed

You can:

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