"Permissions" keys with $SHELL variable format do not get subsituted.

[jsoverson]

Describe the bug
The permissions field for external collections does not undergo env variable substitution.

Actual behavior
Using something like "/": "$PWD" results in an error within the wasmtime provider because $PWD does not exist.

Expected behavior
Shell-like variables should get replaced the same way across all parts of the manifest.

To Reproduce
Try to run any manifest with a permissions block like:

permissions:
  dirs: 
    `"/": "$PWD"`

Environment:

• OS: Mac
• Architecture: m1
• Build Version: r1

Additional context
Add any other context about the problem here.