Move default TEST_OS to centos-8-stream

We don't run centos-7 in CI any more, and thus can't guarantee that it
works.

Makefile

@@ -2,7 +2,7 @@
 PACKAGE_NAME := $(shell awk '/"name":/ {gsub(/[",]/, "", $$2); print $$2}' package.json)
 VERSION := $(shell T=$$(git describe 2>/dev/null) || T=1; echo $$T | tr '-' '.')
 ifeq ($(TEST_OS),)
-TEST_OS = centos-7
+TEST_OS = centos-8-stream
 endif
 export TEST_OS
 TARFILE=cockpit-$(PACKAGE_NAME)-$(VERSION).tar.gz