Use something else than --no-trunc in make -C gadgets sign

[eiffel-fl]

          I also had this issue in the CI and I added the extra space in `"$$GADGET "` to cope with this.

Sadly, this --no-trunc is really a workaround as we do not have JSON and I did not want to add extra dependencies on this PR.
The show/inspect could also be welcomed.

I will think about it, but clearly I will not let this --no-trunc here forever.

Originally posted by @eiffel-fl in #2678 (comment)