WCOW: local and tar exporters on Windows failing with error: "Access is denied."

[profnandaa]

When you use --output type=local, it fails during file copy operation.

buildctl build `
 --frontend=dockerfile.v0 `
 --local context=... `
 --local dockerfile=... `
 --output type=local,dest=C:\dev\tmp `
 --progress plain --no-cache

...
#6 [2/2] COPY hello.txt C:\
#6 DONE 0.5s

#7 exporting to client directory
#7 copying files
#7 copying files 6.30kB 0.0s done
#7 ERROR: open C:\Users\nandaa\AppData\Local\Temp\buildkit-mount1788466376\System Volume Information: Access is denied.

---

Stack trace:

time="2024-06-03T16:09:54+03:00" level=error msg="/moby.buildkit.v1.Control/Solve returned error: rpc error: code = Unknown desc = open C:\\Users\\nandaa\\AppData\\Local\\Temp\\buildkit-mount1788466376\\System Volume Information: Access is denied."
open C:\Users\nandaa\AppData\Local\Temp\buildkit-mount1788466376\System Volume Information: Access is denied.
27908 v0.0.0+unknown C:\dev\container-core\buildkit\cmd\buildkitd\__debug_bin16257889.exe -debug
github.com/moby/buildkit/session/filesync.sendDiffCopy
        C:/dev/container-core/buildkit/session/filesync/diffcopy.go:25
github.com/moby/buildkit/session/filesync.CopyToCaller
        C:/dev/container-core/buildkit/session/filesync/filesync.go:365
github.com/moby/buildkit/exporter/local.(*localExporterInstance).Export.func1.1
        C:/dev/container-core/buildkit/exporter/local/export.go:158
golang.org/x/sync/errgroup.(*Group).Go.func1
        C:/dev/container-core/buildkit/vendor/golang.org/x/sync/errgroup/errgroup.go:75
runtime.goexit
        C:/Program Files/Go/src/runtime/asm_amd64.s:1695

[profnandaa]

Cross-linking #4837 -- has similar error from fs.Walk().