staging: Ensure a clean recipe sysroot removes addto_recipe_sysroot stamps

staging: Ensure a clean recipe sysroot removes addto_recipe_sysroot stamps

The commands:

bitbake nodejs-native;
bitbake nodejs-native -c clean;
bitbake nodejs-native;
bitbake -c addto_recipe_sysroot nodejs-native;
bitbake -c devshell nodejs-native;
bitbake -c addto_recipe_sysroot nodejs-native;
bitbake -c devshell nodejs-native;

never result in npm in the sysroot within devshell. The reason is the
addto_recipe_sysroot stamp isn't removed when do_fetch is run but the sysroot
is cleaned.

With this patch, the second devshell will contain npm, which I think is probably
the best outcome we can hope for here.

[YOCTO #11461]

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