Allow dash in environment variable names

[lchayoun]

Allowed in docker compose and kubernetes:
docker/compose#2373
kubernetes/kubernetes#48986

If filing a bug please include the following:

Nomad version

Nomad v0.9.3 (c5e8b66)

Operating system and Environment details

Not relevant

Issue

Nomad replaces dash with underscore in environment variables

Reproduction steps

Run a job with env stanza the contains a key with dash

Job file (if appropriate)

Not relevant

Nomad Client logs (if appropriate)

Not relevant

Nomad Server logs (if appropriate)

Not relevant

[github-actions]

I'm going to lock this issue because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active issues.
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