The copying and tagging tests force-drop each bucket's collection at the
master so volume slots are freed deterministically between tests. But the
copy-tests CI job runs its master on 9336 and the tagging Makefile on 9338,
while the tests default to 9333 — the cleanup dialed a dead port and quietly
no-oped. Each test bucket then grows 7 volumes against -volume.max=100, and
whenever async deletion lagged the data node ran out of slots and PutObject
500ed with "No writable volumes and no free volumes left".
Set MASTER_ENDPOINT where the master port is non-default: the copy-tests
workflow step, and the copying/tagging Makefiles (derived from MASTER_PORT).