fix: don't break on --substitute values which have commas

[hallyn]

The following:

stacker build --substitute a=b,c

used to work, until commit 3baba644:

fix(ci): convert ci is failing due to perms (#439)

This squashed PR included 'chore(go.mod): update cli to github.com/urfave/cli/v2'. Switching from urfave/cli/v1 to v2 changes string slice flag behavior to automatically split on commas.

Luckily there is an app.DisableSliceFlagSeparator flag we can set to tell it not to do that. Set that flag. And add a test for this.

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (d54e250) 57.10% compared to head (d9cb119) 57.12%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #580 +/- ## ========================================== + Coverage 57.10% 57.12% +0.01% ========================================== Files 64 64 Lines 7519 7521 +2 ========================================== + Hits 4294 4296 +2 Misses 2483 2483 Partials 742 742 ```

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[hallyn]

not ok 66 import various sizes in 4sec

@rchincha is this a regression in this test? (i don't see how it could be, but...) Or has it been seen elsewhere?