nixos/zfs: replace parentheses with braces
nixos/zfs: replace parentheses with braces

The parentheses prevent the `continue` line from working by running the
enclosed in a subshell -- I noticed that ZFS would start asking me for
my password to encrypted child datasets, even though they were not
specified in `requestEncryptionCredentials`. The following logs would
also be present in the import unit's journal:

    Oct 31 22:13:17 host systemd[1]: Starting Import ZFS pool "pool"...
    Oct 31 22:13:44 host zfs-import-pool-start[3711]: importing ZFS pool "pool"...
    Oct 31 22:13:44 host zfs-import-pool-start[4017]:pool/nix/store/39zij3xcxn4w38v6x8f88bx8y91nv0rm-unit-script-zfs-import-pool-start/bin/zfs-import-pool-start: line 31: continue: only meaningful in a `for', `while', or `until' loop
    Oct 31 22:13:44 host zfs-import-pool-start[4020]:pool/nix/store/39zij3xcxn4w38v6x8f88bx8y91nv0rm-unit-script-zfs-import-pool-start/bin/zfs-import-pool-start: line 31: continue: only meaningful in a `for', `while', or `until' loop
    Oct 31 22:15:14 host zfs-import-pool-start[4023]: Failed to query password: Timer expired
    Oct 31 22:15:14 host zfs-import-pool-start[4024]: Key load error: encryption failure
    Oct 31 22:15:14 host systemd[1]: zfs-import-pool.service: Main process exited, code=exited, status=255/EXCEPTION
    Oct 31 22:15:14 host systemd[1]: zfs-import-pool.service: Failed with result 'exit-code'.
    Oct 31 22:15:14 host systemd[1]: Failed to start Import ZFS pool "pool".

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