ui-snapshot: do not access $HOME

It's a bit tedious to have to do this here too. If we encounter other
issues with $HOME down the line, I'll look into adding some nice utility
functions to handle this, or perhaps giving up on the hope that we could
keep $HOME defined for scripts.

This commit additionally adds a test case, should the issue surface
again.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>