[ra_ap_load_cargo] Use `std::fs::canonicalize()` instead of `std::env::current_dir()?.join(..)`

[snprajwal]

In ra_ap_load_cargo::load_workspace_at():

rust-analyzer/crates/load-cargo/src/lib.rs

Lines 47 to 50 in facaf8b

	let root = AbsPathBuf::assert_utf8(std::env::current_dir()?.join(root));
	let root = ProjectManifest::discover_single(&root)?;
	let manifest_path = root.manifest_path().clone();
	let mut workspace = ProjectWorkspace::load(root, cargo_config, progress)?;

We are doing std::env::current_dir()?.join(root) assuming that the path being supplied is relative to the current directory. root.as_ref().canonicalize()? achieves the same thing, but also allows absolute paths to be passed into the function by external consumers of the library API. I think this is a nice improvement that does not break behaviour (as far as I can see). Happy to send a patch if acceptable.

[snprajwal]

std::path::Path::join replaces &self if the provided path is absolute. This already works as expected.