Collect garbage when generating Wasm files

[blaxill]

It appears calling cargo build with a custom target directory is generating a lot of garbage. Either collect the garbage or use the default directory and parse the location from cargo.

[tiziano88]

If the goal of #578 was to make it easier to determine the location of the artifacts, and make sure they are up-to-date, I think it could be achieved without having to rebuild everything and using a separate temp folder. For instance, it may rebuild things in-place (if necessary), and then have some logic to figure out the current target directory. Would this help?

[blaxill]

Rebuilding in place is fine, the default target path depends on the location of the workspace. We could

• Use out-dir instead of target-dir to a temp dir, then clean up after the test (this should reuse intermediate build artefacts)
• Parse the cargo json blob to get the default target directory for the current workspace.