Shared work directories work by assuming bitbake will not run
more than one task with a specific stamp name. Recent runqueue optimisations
accidentally broke this meaning there could be races. This fixes the code.
(Bitbake rev: b1628b1a260ddf43fc9985535b1ddcfcebbb1e5b)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>