Refactor user environment variable handling

[smira]

Code in question: https://github.com/siderolabs/talos/blob/1fb8453c2db1659dd6c1670e4174125b26e777c5/internal/app/machined/pkg/runtime/v1alpha1/v1alpha1_sequencer_tasks.go#L541-L555

Instead, use a controller which will update the environment based on the configuration.

Allow to remove env variables.

In the sequencer, for compatibility need to have a checkpoint which will ensure (via output resources) that controller updated the environment before proceeding with the boot.