Default to --access github

I've been using `cargo-bisect-rustc` for a while now, and `--access
github` has never caused any problems. Maybe it can run into
ratelimiting in extreme cases, but the user should then just use `git`.

Meanwhile, git is a very confusing default as it will suddelny just
start cloning the repo unless you were very careful when installing.

Author: Noratrieb

src/main.rs

@@ -166,7 +166,7 @@ a date (YYYY-MM-DD), git tag name (e.g. 1.58.0) or git commit SHA."
     #[arg(long, help = "Bisect via commit artifacts")]
     by_commit: bool,
 
-    #[arg(long, value_enum, help = "How to access Rust git repository", default_value_t = Access::Checkout)]
+    #[arg(long, value_enum, help = "How to access Rust git repository", default_value_t = Access::Github)]
     access: Access,
 
     #[arg(long, help = "Install the given artifact")]