d/rules: Unset LD_PRELOAD to run unit tests

In particular, this disables faketime(1), as used by reprotest, which
otherwise causes t/git-cgi.t to fail because ikiwiki does not notice
that a file has changed.
6 jobs for debian/master in 11 minutes and 5 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
passed #204277
build

00:04:41

 
  Test
passed #204280
autopkgtest

00:06:15

passed #204281
blhc

00:01:46

passed #204279
lintian

00:02:19

passed #204282
piuparts

00:03:10

passed #204278
reprotest

00:06:24