Allow disabling PRC via env var

[VenelinMartinov]

Ahead of #1785, it might be wise to add this to allow users to debug or work around PRC issues.

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 61.10%. Comparing base (f5a6151) to head (84759f0).
Report is 1 commits behind head on master.

Files	Patch %	Lines
pkg/tfshim/sdk-v2/provider.go	33.33%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2077      +/-   ##
==========================================
- Coverage   61.19%   61.10%   -0.09%     
==========================================
  Files         339      339              
  Lines       45130    45245     +115     
==========================================
+ Hits        27617    27647      +30     
- Misses      15992    16075      +83     
- Partials     1521     1523       +2     

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[iwahbe]

This is not a function. I wasn't sure what the correct "warn" mechanism was here, so I left warn as a placeholder for "warn the user".

[VenelinMartinov]

I don't know either so let's merge as is as this is low-value either way.

This is a debug only env var, primarily to be used by us. If any user is using it, it will be after we've given them the command, so should be ok.

Also BLAH=disable likely doesn't work anywhere.

[iwahbe]

I don't know either so let's merge as is as this is low-value either way.

If there isn't a obvious logging mechanism here, we can just use the standard library's slog.

This is a debug only env var, primarily to be used by us. If any user is using it, it will be after we've given them the command, so should be ok.

I don't want to respond to user's because they typed PULUMI_ENABLE_PLAN_RESOURCE_CHANGE=felse. Even more so, I don't want to spend time because I typed felse instead of `false.

Also BLAH=disable likely doesn't work anywhere.

BLAH=disable currently works for PULUMI_ENABLE_PLAN_RESOURCE_CHANGE.