rustpkg: Make checked-out source files read-only, and overhaul where temporary files are stored

r? @metajack rustpkg now makes source files that it checks out automatically read-only, and stores
them under build/.

Also, refactored the `PkgSrc` type to keep track of separate source and destination
workspaces, as well as to have a `build_workspace` method that returns the workspace
to put temporary files in (usually the source, sometimes the destination -- see
comments for more details).

Closes #6480