Fix knitr 1.44 breakage by unlocking opts_current before modifyfing it

[cderv]

This will solve #209

As described in https://yihui.org/en/2023/09/knitr-1-44/ there is a change with opts_current that requires to unlock if modified for good reason, as the object is meant to be read-only

This PR does that. In general, there is other better ways to modify options in knitr and this should be targeted but this will solve current problems to update on CRAN.

I did not added some tests because it would require Plots package and not sure how you handle Julia deps in there.

The chunk that causes issue is :

```{julia}
using Plots
plot(x->x)
```