Make a consistent use of development rustc invocations

[spastorino]

Here https://rustc-dev-guide.rust-lang.org/compiler-debugging.html we do:

./build/x86_64-unknown-linux-gnu/stage1/bin/rustc in some places and rustc +local in some others.

I'd propose to use consistently rustc +stage1 which if I'm not wrong it's what we explain how to setup at the beginning of the guide.

[igaray]

@rustbot claim