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 30 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #294370
build

00:02:57

 
  Test
passed #294374
autopkgtest

00:03:30

passed #294375
blhc

00:01:25

passed #294373
lintian

00:01:36

passed #294376
piuparts

00:03:18

passed #294372
test-build-all

00:03:27

passed #294371
test-build-any

00:02:10