buildstream/plugins/sources/local.py: Make staging deterministic.

Instead of copying metadata on files staged by local, we manually set
mode to 0755 or 0644 depending on whether user execution was enabled
on source file.

This makes file modes deterministic independently on the way source
was distributed.

Non-deterministic mode copying all metadata can still be enabled by
disable 'deterministic' Boolean configuration on the plugin.

Fixes #527.