ceph-pull-requests-arm64: only build on bionic

ceph-pull-requests-arm64: only build on bionic

ktdreyer/ceph-el8 is amd64 only, so to avoid failures like:

```
No matching package to install: 'python3-cherrypy'
No matching package to install: 'python3-pecan'
No matching package to install: 'python3-routes'
```

we need to disable the "make check" test on the combination of
"arm64+centos8" until these packages are packaged for arm64 or
provided by centos8.

Signed-off-by: Kefu Chai <kchai@redhat.com>