tests/mutation_reader: do not capture on-stack variable by reference
tests/mutation_reader: do not capture on-stack variable by reference

'shard' is a short-lived on-stack variable that gets captured by
reference by continuation that gets executed on another shard.

Fixes a race condition that leads to an heap-use-after-free.

Message-Id: <6cd4507f8ce01d22fdd05d470682f1eaee60e67a@scylladb.com>

avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla