receive: ensure file path matches os-style form when registering

[crazy-max]

fixes docker/buildx#2207
related to docker/buildx#2207 (comment)

When sending files, path will be sent through the wire and treated as-is on the receiver side. We need to make sure receiver side matches its local form when registering. This is specially important on Windows where filepath.FromSlash on sender side converts / into \.

cc @jedevc @gabriel-samfira

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 40.00000% with 3 lines in your changes are missing coverage. Please review.

❗ No coverage uploaded for pull request base (master@b847aa4). Click here to learn what that means.

Files	Patch %	Lines
receive.go	40.00%	1 Missing and 2 partials ⚠️

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Additional details and impacted files

@@            Coverage Diff            @@
##             master     #186   +/-   ##
=========================================
  Coverage          ?   64.51%           
=========================================
  Files             ?       29           
  Lines             ?     1939           
  Branches          ?        0           
=========================================
  Hits              ?     1251           
  Misses            ?      453           
  Partials          ?      235           

Flag	Coverage Δ
go-1.20	64.65% <40.00%> (?)
go-1.21	64.51% <40.00%> (?)
noroot	62.88% <40.00%> (?)
root	64.27% <40.00%> (?)
unit	64.51% <40.00%> (?)

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