🐛 escape / defer interpolation of TRIMMED/RESOLVED variables to runtime

[rawkode]

What this PR does / why we need it:

The envsubst needs us to escape the variables that won't exist at template render time with $$

[rawkode]

/assign @detiber

[detiber]

/lgtm
testing this change locally as well

[displague]

I ran into the same problem when running clusterctl config through the quickstart.

I assumed I could set TRIMMED_KUBERNETES_VERSION and RESOLVED_KUBERNETES_VERSION to v1.18.2, and I did, and this lead to Equinix Metal devices being provisioned. The problem was that kubectl and related tools were not installed.

These variables should have been set to 1.18.2 and 1.18.2-00 respectively.

This PR looks like it will fix that.

[gianarb]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

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