fix(toolkit-lib)!: fromAssemblyBuilder mutates globally shared process.env

[rix0rrr]

The Toolkit communicates with the CDK app passing environment variables (containing context, and others).

If the CDK app is executed as a subprocess it is trivial to set those environment variables when executing the subprocess; but for an in-memory synthesis as allowed by fromAssemblyBuilder mutating process.env is not the correct approach: it is clobbering global state, and multiple async assembly builders might fight with each other over the single shared global object.

In fromAssemblyBuilder(), we now pass an additional env prop containing the environment variables we didn't apply. There were already parametes for outdir and context; those must now be passed to an App instantiated inside the builder (whereas previously the App would pick up on the globally mutated process.env changes).

The rest of this PR is reorganizing the code to make the code that touches the environment return values instead of directly applying them.

• Remove withEnv() and withContext().
• Move logging of the final context object out to the call site instead of the builder function.

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.42857% with 1 line in your changes missing coverage. Please review.

Project coverage is 79.20%. Comparing base (2b92bd5) to head (9edc071).

Files with missing lines	Patch %	Lines
packages/aws-cdk/lib/cxapp/exec.ts	96.42%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #528      +/-   ##
==========================================
+ Coverage   78.92%   79.20%   +0.27%     
==========================================
  Files          46       46              
  Lines        6980     6957      -23     
  Branches      777      777              
==========================================
+ Hits         5509     5510       +1     
+ Misses       1452     1427      -25     
- Partials       19       20       +1     

Flag	Coverage Δ
suite.unit	79.20% <96.42%> (+0.27%)	⬆️

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[rix0rrr]

TODO: this pessimizes the default API for an edge case.

Retain all the code changes, but make it an option; leave the legacy behavior by default. This will require capturing the OUTDIR and CONTEXT envvars in the env bundle to fromAssemblyBuilder() as well.