Only check that changeDirectory() works if HOME exists

[d-torrance]

It may not in some build environments (e.g., Ubuntu PPA builds).

This is a quick followup to #3269 after some of the PPA builds started failing:

../../m2/debugging.m2:18:6:(1):[9]: error: installPackage: 1 error(s) occurred running examples for package Macaulay2Doc:

_change__Directory.errors
*************************
i3 : changeDirectory dir

o3 = /tmp/M2-24422-0/0/

i4 : currentDirectory()

o4 = /tmp/M2-24422-0/0/

i5 : changeDirectory()
stdio:5:1:(3): error: changing directory failed: No such file or directory

(full log)