Windows: Cabal < 3.18 `copy` command cannot handle long path if relative

[mpilgrem]

See:

• Add Backpack support to Stack #6865
• cabal crashes in HLS CI while copying files haskell/cabal#11395 (comment)

On Windows, the copy command of Cabal < 3.18 cannot handle long paths that are specified as relative paths.

However, Stack passes --builddir= to Cabal as a relative path. See withCabal of Stack.Build.ExecuteEnv.withSingleContext and its setupArgs.