build.py: Ensure shared work siginfo files are written to the correct location

Right now shared work signature data is saved to the non-shared directory
which is confusing to everyone including bitbake. Whilst its messy,
extra the stampbase data instead, which ensures the sig data is written
to the correct location alongside its corresponding stamp file.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>