fix windows path for dwds/web builds

[jonahwilliams]

Description

This should be the platform separator.

[fluttergithubbot]

It looks like this pull request may not have tests. Please make sure to add tests before merging. While there are exceptions to this rule, if this patch modifies code it is probably not an exception.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@a785db7). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master   #37658   +/-   ##
=========================================
  Coverage          ?   55.25%           
=========================================
  Files             ?      193           
  Lines             ?    18144           
  Branches          ?        0           
=========================================
  Hits              ?    10025           
  Misses            ?     8119           
  Partials          ?        0

Flag	Coverage Δ
#flutter_tool	55.25% <100%> (?)

Impacted Files	Coverage Δ
packages/flutter_tools/lib/src/web/web_fs.dart	24.24% <100%> (ø)

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