Auto merge of #6679 - flip1995:ra_setup_abs_path, r=Manishearth

Use absolute path to Rust repo in ra_setup

This will convert the path to the Rust repo to an absolute path. This is
important for the clippy_lints/Cargo.toml file. Otherwise if a relative
path is passed, rst-analyzer won't find the Rust repo, because it starts
the relative path search from the clippy_lints dir, not the
rust-clippy dir where the ra_setup command was run from.

changelog: none