ci: centos: run on centos stream 8 image from

Uri Lublin requested to merge uril/spice:ci_centos_stream_8 into master

The default centos:latest image can not install packages anymore. The makecheck-centos test fails with the following error: $ dnf install -y 'dnf-command(config-manager)' Error: Failed to download metadata for repo 'appstream'
Cannot prepare internal mirrorlist: No URLs in mirrorlist

Replace it with a centos stream 8 image from to make the makecheck-centos test pass

Signed-off-by: Uri Lublin

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