Fix building of rust extensions in dist mode
Fix building of rust extensions in dist mode

Rust extensions are currently forcibly built in-place, because they
might be imported by further build phases.  This, however, breaks dist
builds, because the .so is actually not copied to build/.  Fix this by
emulating `inplace` instead of actually forcing it.

Closes: #1166.

edgedb/edgedb,edgedb/edgedb,edgedb/edgedb