Fix qemu driver filesystem variable interpolation

[mismithhisler]

Description

There was no good way to reference an image located in the NOMAD_ALLOC_DIR. Relatives paths did not work (this does not fix that), and the NOMAD_ALLOC_DIR envvar was set to /alloc which is used in containers. These changes set the filesystem envVars to host paths so they can be interpolated correctly in a job spec.

Testing & Reproduction steps

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Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

[tgross]

LGTM!

Where this might cause some quirky behavior is if the task driver were to expose the task dir and allocdir to the task with image isolation. But as far as I know there's no existing task driver that does this. A reasonable approach for the task driver to support this would be to expose it as a sshfs or something like that, but at that point the task driver needs to know the real source on disk so it'd be responsible for doing any environment variable mapping in the userdata it injects. (Mostly leaving this rambling comment so that if we wonder about it in the future we have a record of why 😁 )

[mismithhisler]

@tgross good points that I didn't consider, thanks!