Verify that nmcli installed

[mcm001]

Description

Previously, NetworkManager would happily go asking for networkmanager to do things even if it wasn't installed. This should never be the case, but we should bail out early if it is regardless IMO. This prevents logs and the UI from looking suspiciously "working", if you ignore the exit code. Up for debate if we actually need this feature.

Meta

Merge checklist:

• Pull Request title is short, imperative summary of proposed changes
• The description documents the what and why
• If this PR changes behavior or adds a feature, user documentation is updated
• If this PR touches photon-serde, all messages have been regenerated and hashes have not changed unexpectedly
• If this PR touches configuration, this is backwards compatible with settings back to v2024.3.1
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• If this PR addresses a bug, a regression test for it is added