MAINT: Get `R CMD check` to give less notes

[HaoZeke]

I see that R CMD check seem to give a note about pixi files, it seems pixi.* might not
be the correct syntax in .Rbuildignore? (I do not know what the correct syntax would be,
but the check gives a note both in github actions and locally it seems. If all else fails, adding the files one by one instead of a wildcard syntax definitely works)

First noted here: ropensci/software-review#606 (comment).