mkosi: trim verity.sig json files to remove NUL padding before passing to jq (#41767)

jq started rejecting input that has NUL bytes to fix some security
issues,
so we need to trim the verity.sig json files, which are spat out with
the NUL bytes padding from the GPT partition content.

```
‣  Running postinstall script /home/runner/work/systemd/systemd/mkosi/mkosi.postinst.chroot…
jq: parse error: Invalid numeric literal at EOF at line 1, column 16384
‣ "/work/postinst final" returned non-zero exit code 5.
```

https://github.com/jqlang/jq/commit/6374ae0bcdfe33a18eb0ae6db28493b1f34a0a5b

https://github.com/systemd/systemd/pull/41771 updates the producer.