fly: fix use of zstd reader in tests
fly: fix use of zstd reader in tests

these fly integration tests were not update when we switched from the
cgo-based zstd library to the new go-baseed one in #5024.

Signed-off-by: Ciro S. Costa <4f5054ab2db051d03adf9068233a850c82b24437@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse