In commit 405fcbc1e ("pveceph: switch repo sources to modern deb822
format") we switched away from single-line repos, but the later
rebased commit 9c0ac59e0 ("fix #5244 pveceph: install: add new
repository for offline installation") seemingly missed that change and
was not re-tested, thus it referenced the previous variable name and
old file ending, as this was already pushed let's fix it as this
follow-up.
Fixes: 9c0ac59e0 ("fix #5244 pveceph: install: add new repository for offline installation")
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>