CI: Disable reprotest job, as it leads to false positives

The reprotest job is still quite buggy, and leads to false positives, as
it doesn't use the same setup as the "official" one which is run on
tests.reproducible-builds.org. Accordingly, for now, disable it and
revisit this decision in the future.
7 jobs for master in 6 minutes and 55 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #294453
build

00:03:04

 
  Test
passed #294457
autopkgtest

00:03:49

passed #294458
blhc

00:01:32

passed #294456
lintian

00:01:42

passed #294459
piuparts

00:03:18

passed #294455
test-build-all

00:02:56

passed #294454
test-build-any

00:02:13