Fix CPack package naming when cross-compiling

[alexreinking]

I expect this to work, but I need to get into the armbots to test it.

[alexreinking]

Looks like ccache on one of the bots got messed up and there's a new LLVM 20 failure. This code doesn't run on the normal buildbots, so must be unrelated.

[steven-johnson]

LGTM pending green

EDIT: this won't actually test the nightlies which are affected, and there isn't a way to easily test the nightlies outside of the main branch, so we'll land this once we get a handful of greens as a realtiy check