Use isolated GNUPGHOME for keycachetest

I have no idea why keycachetest started to fail after seemingly completely
unrelated changes and I couldn't reproduce it locally, but using a temporary
GNUPGHOME for the test seems to fix the failures. It's anyway better to use
isolated test environments.

(cherry picked from commit 938f078e7f6e1dfc0bc852dfbfb534d978192cde)

• GnuPG-bug-id: T8245