sandbox: use sane access rights for files

When writing an executable, allowing other users to modify it introduces
a security issue.

Generally we should avoid giving other users write access to our files by
default.

Replace chmod(777) by chmod(755) and chmod(644).

Fixes: 47f5fcfb ("sandbox: Add os_jump_to_image() to run another executable")
Fixes: d9165153 ("sandbox: add flags for open() call")
Fixes: 5c2859cd ("sandbox: Allow reading/writing of RAM buffer")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Sean Anderson <seanga2@gmail.com>