Fix the traceshift undefined variable bug

[segasai]

Fixes the #2427
Sorry for introducing it.
(can't test it myself, but I think it should do it).

[coveralls]

coverage: 39.44% (-0.003%) from 39.443%
when pulling e12795b on fix_traceshift
into 3c6b692 on main.

[julienguy]

I tested the fix in the context of a nightwatch run of an arc exposure (which crashes with main and works with this branch):

nightwatch run -n 20241211 -e 00267785 --cameras b0,r0,z0 -o .

Merging.