make-disk-image: create directories images in out path

[Mic92]

In case we are crossing some filesystem boundary (i.e. /tmp -> /nix/store), this is faster. Also tmpfs might be probelemantic regarding memory usage.

[iFreilicht]

I don't quite understand why this is faster, but I'll trust you tried it out. However, changing the output directory should be considered a breaking change, right?
How do we deal with this? I don't want to break people's automation scripts that rely on the disk images being created in the current directory instead of out.

[Mic92]

If you do a mv across different filesystems, it's actually a copy. It depends a bit on the build machine if this is the case. If both build directory and nix store are on the same FS than it's a cheap operation.

[Mic92]

However, changing the output directory should be considered a breaking change, right?

No this change is not visible to the end-user. cfg.extraPostVM was called after the mv happened. And it shouldn't matter what qemu uses at runtime.