Debian: T973: add missing dependency on node-exporter package

[c-po]

Change Summary

Extend commit a0c15a1 ("T973: add basic node_exporter implementation") by adding the required dependency to install node-exporter binary.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Code style update (formatting, renaming)
• Refactoring (no functional changes)
• Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• Other (please describe):

Related Task(s)

• https://vyos.dev/T973

Related PR(s)

• T973: add basic node_exporter implementation #4048

Checklist:

• I have read the CONTRIBUTING document
• I have linked this PR to one or more Phabricator Task(s)
• I have run the components SMOKETESTS if applicable
• My commit headlines contain a valid Task id
• My change requires a change to the documentation
• I have updated the documentation accordingly

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