Auto-detect when to use GovCloud layer

[DylanLovesCoffee]

What does this PR do?

Removes the isGovCloud parameter in favor of automatically detecting when a user is deploying to GovCloud as to attach the GovCloud lambda layer.

Motivation

Testing Guidelines

Unit tests.

Additional Notes

Types of changes

• [ ] Bug fix
• [x] New feature
• [ ] Breaking change
• [ ] Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• [x] This PR's description is comprehensive
• [ ] This PR contains breaking changes that are documented in the description
• [ ] This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• [ ] This PR impacts documentation, and it has been updated (or a ticket has been logged)
• [x] This PR's changes are covered by the automated tests
• [ ] This PR collects user input/sensitive content into Datadog

[codecov-io]

Codecov Report

Merging #17 into master will decrease coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #17      +/-   ##
==========================================
- Coverage   88.69%   88.66%   -0.04%     
==========================================
  Files           7        7              
  Lines         345      344       -1     
  Branches       81       81              
==========================================
- Hits          306      305       -1     
  Misses         20       20              
  Partials       19       19              

Impacted Files	Coverage Δ
serverless/src/env.ts	94.44% <ø> (ø)
serverless/src/index.ts	90.19% <100.00%> (ø)
serverless/src/layer.ts	98.03% <100.00%> (-0.04%)	:arrow_down:

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