fool build process into thinking sources are mounted with write permissions in order to work around the copy patch

[Atemu]

We have a patch for many generic copy functions to not copy the permissions as all permissions of the sources are the default nix store permissions. This patch has to be rebased for every major android version though and isn't the best of solutions.

An idea I had on this was to somehow fool the build system into thinking the source dirs have write permissions via some mount magic.