mkosi: set file permissions in copy of source tree (#8370)

mkosi: set file permissions in copy of source tree (#8370)

Meson keeps permissions around during the build, so details of how umask
was set when cloning the original git tree will leak all the way to the
installed files in the mkosi image.

So reset the permissions of the files in the copy of the tree before
starting the build.

Also set the umask explicitly.

Tested by creating a mkosi image and booting it on a tree that was
cloned with a umask of 027, confirmed that the *.target files were not
created as world-unreadable anymore.