Fix ReadAll to run on Windows.
Fix ReadAll to run on Windows.

filepath.Clean converts filenames to filenames with native path
separators. Use ToSlash to normalize.

Signed-off-by: Anusha Ragunathan <11258be80bf4b196e47582a3113274a53ec55793@docker.com>

tonistiigi/buildkit_poc,tonistiigi/buildkit_poc