Fix preserveModulesRoot path on Windows

[lukastaegert]

This PR contains:

• bugfix
• feature
• refactor
• documentation
• other

Are tests included?

• yes (Existing tests were red on Windows) (bugfixes and features will not be merged without tests)
• no

Breaking Changes?

• yes (breaking changes will not be merged unless absolutely necessary)
• no

List any relevant issue numbers:

Description

Unfortunately as Windows tests do not run on forks, there was a Windows issue that was not noticed

[rollup-bot]

Thank you for your contribution! ❤️

You can try out this pull request locally by installing Rollup via

npm install rollup/rollup#fix-windows-path

or load it into the REPL:
https://rollupjs.org/repl/?circleci=13030

[codecov]

Codecov Report

Merging #3791 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3791   +/-   ##
=======================================
  Coverage   97.03%   97.03%           
=======================================
  Files         185      185           
  Lines        6478     6478           
  Branches     1877     1877           
=======================================
  Hits         6286     6286           
  Misses        101      101           
  Partials       91       91           

Impacted Files	Coverage Δ
src/Chunk.ts	100.00% <100.00%> (ø)

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