sstate: another fix for touching files inside pseudo

This patch is a fixup for 676757f "sstate: fix touching files inside pseudo"

running the 'id' command inside the sstate_unpack_package
function shows that this funcion run inside the pseudo:

 uid=0(root) gid=0(root) groups=0(root)

The check for [ -w ${SSTATE_PKG} ] and [ -O ${SSTATE_PKG}.siginfo ]
will always return true and the touch can fail when the real user
don't have permission or in readonly filesystem.

As the documentation refers:
- the file test operator "-w" check if the file has write permission
(for the user running the test).
- the file test operator "-O" check if you are owner of file

We can avoid this test running the touch and mask any return errors
that we have.

(From OE-Core rev: 29fc8599)

Signed-off-by: Jose Quaresma <quaresma.jose@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 5b9210d66c78bb3f79056e5586cea7b0edd714a9)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>