fix(cook): copy source only if newer

Fixes the bug where libgcc was unnecessarily building multiple times.
Which in turn caused other packages to rebuild.

Example: The source path is set to `prefix/${TARGET}/sysroot`. Currently
the way `cook` handles `source.path` ends up messing the timestamp of
`recipes/libs/libgcc/source` as it just performs `copy_dir_all(path,
&source_dir)` without checking if the timestamp of the source path
specified in the recipe was changed. This would consequently update the
timestamp of the source and cause it to rebuild.

Signed-off-by: Anhad Singh <andypython@protonmail.com>