nixos/systemd: update max line length in systemd units

The length check was introduced[0] to match systemd's max line
length. This limit has been increased[1][2] to 1MiB, starting with
systemd v235.

[0] https://github.com/nixos/nixpkgs/issues/3403
[1] https://github.com/systemd/systemd/commit/e6dde451a51dc5aaa7f4d98d39b8fe735f73d2af
    (relevant systemd commit)
[2] https://github.com/systemd/systemd/issues/3302
    (more context on systemd change)