Add subscription id to azure quickstart

[rmweir]

What this PR does / why we need it:
Adds step to export AKS subscription ID env var, since export AZURE_SUBSCRIPTION_ID_B64="$(echo -n "$AZURE_SUBSCRIPTION_ID" | base64 | tr -d '\n')" won't work without it

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 #

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[k8s-ci-robot]

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[rmweir]

/assign @ncdc

[ncdc]

/ok-to-test
/lgtm
/approve
/kind documentation

thank you!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ncdc, rmweir

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