fix: don't reset context.scratch between files

[zsol]

fix: don't reset context.scratch between files

#453 fixed scratch leaking between files by setting it to empty, but that drops all the scratch space that was set up before the codemod runs (e.g. in the transformer's constructor)

This PR improves the fix by preserving the initial scratch.

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 91.16%. Comparing base (71b0a12) to head (6adb8c3).

Files	Patch %	Lines
libcst/tests/test_e2e.py	83.33%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1151      +/-   ##
==========================================
- Coverage   91.16%   91.16%   -0.01%     
==========================================
  Files         262      262              
  Lines       26836    26842       +6     
==========================================
+ Hits        24465    24470       +5     
- Misses       2371     2372       +1     

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