raw_exec and other unisolated drivers have buggy interpolation in templates

[schmichael]

Nomad version

Nomad v0.12.8 (b850164)

Issue

Interpolation for NOMAD_ALLOC_DIR, NOMAD_TASK_DIR, and NOMAD_SECRETS_DIR is broken for template stanzas when using task drivers like raw_exec which do not have any filesystem isolation.

The way the NOMAD_*_DIR environment variables are initialized depends on whether or not the task driver supports filesystem isolation:

https://github.com/hashicorp/nomad/blob/v0.12.8/client/allocrunner/taskrunner/task_dir_hook.go#L77-L98

However, when interpolating these variables for templating we improperly handle them and cause them to be broken/unusable.

See https://github.com/hashicorp/nomad/pull/9383/files#r526105415 and the associated PR description.

Reproduction steps

In an agent with raw_exec enabled (or the dev agent):

nomad run https://gist.githubusercontent.com/schmichael/2a9132537956c6c764461bb59671c45c/raw/cdf8d11b10c5db4d34e2bee3dff228aaf26719d7/rawexecrepro.hcl

Job file (if appropriate)

https://gist.github.com/schmichael/2a9132537956c6c764461bb59671c45c

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