Fix bug when `cargo-contract` installation had been interrupted

Sergej Kostjucenko requested to merge cmichi-fix-dirty-directory into master

Created by: cmichi

Fixes https://github.com/paritytech/cargo-contract/issues/513.

If the crate installation had been interrupted the file _Cargo.toml had already been renamed to Cargo.toml. If the installation was resumed the _Cargo.toml was no longer found. This resulted in the panic Failed renaming _Cargo.toml to Cargo.toml: File not found..

I fixed it by copy (with overwrite) the file _Cargo.toml to Cargo.toml instead.

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