copy: Once again, ensure pwrite scripts consume all data

I saw a transient test failure similar to one we previously tried to
fix in c713529e; this time on Debian 11:

+ nbdcopy --allocated --request-size=32768 -- '[' nbdkit --exit-with-parent data 'data=
             1
             @1073741823 1
             @4294967295 1
             @4294967296 1
             ' ']' '[' nbdkit --exit-with-parent eval 'get_size= echo 7E ' 'pwrite= echo $@ >> copy-sparse-allocated.out ' 'trim= echo $@ >> copy-sparse-allocated.out ' 'zero= echo $@ >> copy-sparse-allocated.out ' ']'
libnbd: debug: nbd1: nbd_create: opening handle
libnbd: debug: nbd1: nbd_set_debug: enter: debug=false
libnbd: debug: nbd2: nbd_create: opening handle
libnbd: debug: nbd2: nbd_set_debug: enter: debug=false
nbdkit: eval[1]: error: /tmp/nbdkitcZwROL/pwrite: write: Broken pipe
nbdkit: eval[1]: error: /tmp/nbdkitcZwROL/pwrite: script exited with error, but did not print an error message on stderr
nbdcopy: write at offset 4294967296 failed: Input/out...