Mount helm login secret in arbitrary location to prevent conflict with default helm behaviour

[rain-on]

Background

The agent upgrade credentials secret is mounted directly into /root/.config/helm/registry/config.json.

This makes the container directory READ ONLY.

Thus - if a job attempts to manually perform a "helm registry login" - it fails, as the config.json file is also readonly (regardless of if the secret is defined or not).

This change mounts the secret in an arbitrary location (/root/agent_upgrade/config.json), such that default helm operations continue to operate as expected.

For tasks to use the mounted secret, they are expected to export "HELM_REGISTRY_CONFIG=/root/agent_upgrade/config.json" prior to making a helm call.

Results

Fixes https://github.com/OctopusDeploy/Issues/issues/... (optional public issue)

Fixes https://github.com/OctopusDeploy/ResearchAndDevelopment/issues/... (optional private issue)

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