pipeline_definitions: Don't build and push images and charts on pull requests

[ialidzhikov]

How to categorize this PR?

/area delivery /kind cleanup

What this PR does / why we need it: After https://github.com/gardener/gardener-extension-registry-cache/pull/229 we noticed that we unnecessary build and push images and charts on pull requests. This PR adapts the extension to be like the gardener/gardener repo: there is already a prow job on PRs and builds the images without pushing it.

Which issue(s) this PR fixes: N/A

Special notes for your reviewer: N/A

Release note:

NONE

[gardener-prow[bot]]

LGTM label has been added.

Git tree hash: acb1b6b14e3d0b65bf222005934c162aafad2d0c

[gardener-prow[bot]]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dimitar-kostadinov

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files: - ~~[OWNERS](https://github.com/gardener/gardener-extension-registry-cache/blob/main/OWNERS)~~ [dimitar-kostadinov] Approvers can indicate their approval by writing `/approve` in a comment Approvers can cancel approval by writing `/approve cancel` in a comment

[ialidzhikov]

/override concourse-ci/helmcharts

[gardener-prow[bot]]

@ialidzhikov: Overrode contexts on behalf of ialidzhikov: concourse-ci/helmcharts

In response to [this](https://github.com/gardener/gardener-extension-registry-cache/pull/258#issuecomment-2357551182): >/override concourse-ci/helmcharts Instructions for interacting with me using PR comments are available [here](https://git.k8s.io/community/contributors/guide/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes-sigs/prow](https://github.com/kubernetes-sigs/prow/issues/new?title=Prow%20issue:) repository.