special case when doing build-obj with just one source file

When building an object file from only one source file, instead of
having a two-stage cache system, we special case it and use the cache
directory that the .o file is output to as the final cache directory for
all the build artifacts.

When there are more than 1 source file, the linker has to merge objects
into one, and so the two stage approach makes sens. But in the case of
only one source file, this prevents needlessly copying the object file.

This commit fixes an issue with the previous one, where zig with cache
enabled would print a directory that actually did not have any build
artifacts in it.

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