Fix istio no-mesh build command

[ReToCode]

Which issue(s) this PR fixes:
Fixes knative/serving#15274

What this PR does, why we need it:

• Fixes istio build config (no-mesh build should have --no-mesh)

[ReToCode]

/assign @dprotaso
/assign @skonto

[dprotaso]

/lgtm
/approve

[knative-prow]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: dprotaso, ReToCode

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 [dprotaso]

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

[knative-prow]

@ReToCode: Updated the job-config configmap in namespace default at cluster default using the following files:

• key serving-main.gen.yaml using file prow/jobs/generated/knative/serving-main.gen.yaml

In response to this:

Which issue(s) this PR fixes:
Fixes knative/serving#15274

What this PR does, why we need it:

• Fixes istio build config (no-mesh build should have --no-mesh)

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.