rustpkg: test executables are lost when RUST_PATH is not set

[omasanori]

When we have a package foo in ~/work/foo, an error will occur with below:

$ cd ~/work/foo
$ mkdir -p ~/.rust/src
$ ln -sf ~/work/foo ~/.rust/src
$ unset RUST_PATH
$ rustpkg test foo

but will not if the last command is:

$ RUST_PATH=$HOME/.rust rustpkg test foo

The error message is "error: Internal error: test executable for package ID foo-0.0 in workspace /home/YOUR_NAME/work/foo/.rust wasn't built! Please report this as a bug."

[flaper87]

Closing rustpkg is deprecated