repo-install-test: fix nftables errors on centos8

Don't attempt to install nftables-devel or python3-nftables in centos8
in the repo-install-test. What happens is, that dnf tries to install the
versions from the centos repositories, which conflict with the ones from
the OBS repositories. This fixes the failing
Osmocom-repo-install-centos8 jenkins job.

Change-Id: I57670069cc09a5493667971997f0245e1d6a7259

scripts/repo-install-test/blacklist_centos8.txt

@@ -9,3 +9,7 @@
 # the dependency problem means that we can't simply install the package in
 # centos. In OBS we work around this in the project config.
 ulfius-devel
+
+# Also linked to opensuse versions
+nftables-devel
+python3-nftables