Fix Windows build for finding the cargo executable

[Xmader]

PR #386 tried to make the cargo executable immediately available in the current shell by sourcing $HOME/.cargo/env.

console messages when cargo is installed by rustup on *nix

Rust is installed now. Great!

To get started you may need to restart your current shell.
This would reload your PATH environment variable to include
Cargo's bin directory ($HOME/.cargo/bin).

To configure your current shell, you need to source
the corresponding env file under $HOME/.cargo.

This is usually done by running one of the following (note the leading DOT):
. "$HOME/.cargo/env"            # For sh/bash/zsh/ash/dash/pdksh
source "$HOME/.cargo/env.fish"  # For fish

However, this method does not work in Windows, and thus breaks our Windows build CI.

console messages when cargo is installed by rustup on Windows

Rust is installed now. Great!
To get started you may need to restart your current shell.
This would reload its PATH environment variable to include
Cargo's bin directory (%USERPROFILE%\.cargo\bin).

Error

 ./setup.sh: line 37: /c/Users/runneradmin/.cargo/env: No such file or directory

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This PR fixes this error by executing cargo by its absolute path.

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Closes #408