update machineType in cloudbuild

[Prajyot-Parab]

What this PR does / why we need it:

• update machineType in cloudbuild
• Fix buildx platform list

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:

/area provider/ibmcloud

1. Please confirm that if this PR changes any image versions, then that's the sole change this PR makes.

Release note:

- update machineType in cloudbuild
- Fix buildx platform list

[netlify]

✅ Deploy Preview for kubernetes-sigs-cluster-api-ibmcloud ready!

Name	Link
🔨 Latest commit	ed17c96
🔍 Latest deploy log	https://app.netlify.com/sites/kubernetes-sigs-cluster-api-ibmcloud/deploys/6466f3166eafc00008a75253
😎 Deploy Preview	https://deploy-preview-1233--kubernetes-sigs-cluster-api-ibmcloud.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 site settings.

[mkumatag]

Let's understand what machine type is used by default vs E2_HIGHCPU_8 and the spec.

[Prajyot-Parab]

Let's understand what machine type is used by default vs E2_HIGHCPU_8 and the spec.

e2-medium (default)	1 vCPU	Mem 4GB
e2-highcpu-8            8 vCPU  Mem 8GB

[mkumatag]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: mkumatag, Prajyot-Parab

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 [mkumatag]

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