testlibrary: Correct a wrong string in a debug message

This always printed "setting HOME=(null)" because we didn't set
datadir yet, but it should have been looking at homedir anyway.

Signed-off-by: Simon McVittie <smcv@collabora.com>

Closes: #1760
Approved by: alexlarsson
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f