Move installation script out of cargo-creusot

[Lysxia]

Currently cargo-creusot handles the "installation" of creusot-rustc, with a special command which only works from inside the creusot source repository. It is not very hygienic to leak those installation details into the tool.

Installing creusot should be done by a standalone script.

(Let me recall how I ended up here at the time (#1314). I had a rustup-like architecture in mind (where rustup installs rustc) as a long-term goal. Since I was telling cargo-creusot where to find creusot-rustc I thought it made sense to put the installation logic at the same place. In hindsight, it was a half-baked idea.)