build: Fix configuring depends with cmake

build: Fix target name

The second commit fixes an issue I see when trying to build an individual package that has native dependencies. The first one is trivial, but I don't really understand how to reproduce the issue. For Bitcoin Core, it manifested when configurinbg the libmultiprocess package (which we didn't backport yet)

This is a backport of core#24392 and core#19868

make -C depends qt HOST=x86_64-apple-darwin

Before this commit, this caused errors such as tar: /home/hebasto/guix/GitHub/bitcoin/depends/built/x86_64-apple-darwin16/native_cctools/native_cctools-55562e4073dea0fbfd0b20e0bf69ffe6390c7f97-ee52008b72b.tar.gz: Cannot open: No such file or directory

After this commit, this error goes away (but the crosscompilation for Mac OS still fails for other reasons)