Add wpkgsharebase var

@cm214 In light of commit 4937255e, I've realised we need to have a variable that points at the base of the communal IFS share, i.e. the parent directory of the location that we're currently setting as WPKGSOFTWARE. This is because we have important directories in \\ifs.ch.private.cam.ac.uk\deploy other than software! I've suggested WPKGSHAREBASE

Whilst this theoretically means we could drop WPKGSOFTWARE and instead refer to %WPKGSHAREBASE%\software, every single package will use WPKGSOFTWARE so I think having both of those variables defined for convenience would be useful.