Fix broken mirrorlist URLs from CentOS Stream repos.

The mirrorlist URLs rely on variable replacements from CentOS Stream that are
not valid once we have switched to Rocky Linux.  To fix this we change to the
baseurl instead and fix it up to not rely on those variables that have changed.

migrate2rocky/migrate2rocky.sh

@@ -741,8 +741,12 @@ package_swaps() {
 "Could not remove packages from the rpm db: ${installed_sys_stream_repos_pkgs[@]}"
 	fi
 
-        # Rename the stream repos with a prefix.
-        sed -i 's/^\[/['"$stream_prefix"'/' "${repos_files[@]}"
+        # Rename the stream repos with a prefix and fix the baseurl.
+        sed -i \
+            -e 's/^\[/['"$stream_prefix"'/' \
+            -e 's|^mirrorlist=|#mirrorlist=|g' \
+            -e 's|^#baseurl=http://mirror.centos.org/$contentdir/$stream/|baseurl=|http://mirror.centos.org/centos/8-stream/' \
+	    "${repos_files[@]}"
     fi
 
     # Use dnf shell to swap the system packages out.