run-make: do not use relative links to refer to compiler/stdlib sources, we should introduce some `CHECKOUT_ROOT` env var/helper.

[jieyouxu]

Some run-make tests refer to stdlib sources with "../../../library/alloc/src/lib.rs" which seems not ideal. @Kobzol suggested that we should introduce a CHECKOUT_ROOT helper to make this less fragile.

[bjorn3]

There is already the S env var set by compiletest for the source root.

[jieyouxu]

There is already the S env var set by compiletest for the source root.

I'm actually dumb, we even already have source_path in the support library:

rust/src/tools/run-make-support/src/lib.rs

Lines 90 to 92 in 50297bb

	pub fn source_path() -> PathBuf {
	env_var("S").into()
	}

[ChrisDenton]

That's a very descriptive environment variable name, lol