Add env vars to speed up bisecting tfgen

[t0yv0]

Schema generation takes a long time for large providers like pulumi-aws and adding these helper env vars to limit it to
one problematic resource or data source can be a helpful tool to speed up bisecting over bridge ranges.

[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 62.32%. Comparing base (22980d5) to head (2713bac).

Files	Patch %	Lines
pkg/tfgen/docs.go	0.00%	2 Missing and 1 partial ⚠️
pkg/tfgen/examples_cache.go	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1848      +/-   ##
==========================================
+ Coverage   61.06%   62.32%   +1.26%     
==========================================
  Files         303      309       +6     
  Lines       42432    35612    -6820     
==========================================
- Hits        25910    22195    -3715     
+ Misses      15053    11947    -3106     
- Partials     1469     1470       +1     

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[iwahbe]

This is great for quick iteration.

Can you add a reference to this in docs so this is findable for others? Maybe docs/dev#env vars?

[t0yv0]

Sure