fix image not build issue

[zetxqx]

The original make target does not generate any image, see the following command and result

❯ make image-push
make: Nothing to be done for 'image-push'.

Moving the $(MULTI) to build.yaml instead of putting it in the make target. Because it won't evaluate because
while MULTI is only read at parse time (by the ifdef MULTI block)

However, I didn't verify if the cloud build can generate multiple platform image. We can merge this PR and see the post submit.

[netlify]

✅ Deploy Preview for gateway-api-inference-extension ready!

Name	Link
🔨 Latest commit	69c793b
🔍 Latest deploy log	https://app.netlify.com/projects/gateway-api-inference-extension/deploys/688d89074256a80008209a24
😎 Deploy Preview	https://deploy-preview-1286--gateway-api-inference-extension.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[k8s-ci-robot]

Hi @zetxqx. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

Once the patch is verified, the new status will be reflected by the ok-to-test label.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository.

[danehans]

+1 on merging to see if post submit will build the multi-arch image.

[danehans]

/ok-to-test

[danehans]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: danehans, zetxqx

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 [danehans]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[kfswain]

builds time out now unfortunately