Bump terraform-exec to v0.18.1

[austinvalle]

Background

A recent update of tfexec v0.18.0 caused a regression where detailed stderr information wasn't being passed to downstream modules. This has now been fixed with the new release v0.18.1

Notes

• Added tests for ExpectError, so future CI can catch bugs similar to the one introduced in build(deps): Bump github.com/hashicorp/terraform-exec from 0.17.3 to 0.18.0 #1154
  • Backported from Upgrade tfexec to v0.18.1 + add tests for ExpectError terraform-plugin-testing#86
  • Example of new tests failing w/ tfexec v0.18.0

[bendbennett]

LGTM 👍

[austinvalle]

Skipping the changelog as this regression from #1154 hasn't been released -> 🚀

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