bootstrap/node-image-pull.sh: handle "separate stateroot" flow better

[jlebon]

Some flows currently instead of reinstalling the bootstrap node into a control plane node just add another stateroot and reboot into it. Long- term, those flows should move to a more container-native alternative.

But for now, let's avoid breaking them by deleting the node-image ref so it doesn't leak, and touching the checkout directory so OSTree won't try to GC it.

[openshift-ci]

Skipping CI for Draft Pull Request.
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You can still manually trigger a test run with /test all

[jlebon]

Leaving testing of this to @carbonin.

[openshift-ci]

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[openshift-ci]

@jlebon: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/artifacts-okd-scos-images	1a61121	link	true	/test artifacts-okd-scos-images

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