Update MacOS ISO build steps

[cameronbrunner]

Fixes issue #11902

Two changes required to get MacOS ISO builds working:

1. Include a 'go module' compatible golang environment in the
   buildroot docker image
2. Run the build within a shell environment to have $PWD set

It may be worthwhile to update to Ubuntu 20.04.

[k8s-ci-robot]

Welcome @cameronbrunner!

It looks like this is your first PR to kubernetes/minikube 🎉. Please refer to our pull request process documentation to help your PR have a smooth ride to approval.

You will be prompted by a bot to use commands during the review process. Do not be afraid to follow the prompts! It is okay to experiment. Here is the bot commands documentation.

You can also check if kubernetes/minikube has its own contribution guidelines.

You may want to refer to our testing guide if you run into trouble with your tests not passing.

If you are having difficulty getting your pull request seen, please follow the recommended escalation practices. Also, for tips and tricks in the contribution process you may want to read the Kubernetes contributor cheat sheet. We want to make sure your contribution gets all the attention it needs!

Thank you, and welcome to Kubernetes. 😃

[k8s-ci-robot]

Hi @cameronbrunner. Thanks for your PR.

I'm waiting for a kubernetes 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/test-infra repository.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: cameronbrunner
To complete the pull request process, please assign afbjorklund after the PR has been reviewed.
You can assign the PR to them by writing /assign @afbjorklund in a comment when ready.

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

Needs approval from an approver in each of these files:

• OWNERS

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

[minikube-bot]

Can one of the admins verify this patch?

[sharifelgamal]

I'm cool with updating the base image to ubuntu 20.04, thanks for your help fixing this!