test: remove the use of CARGO_RUSTC_CURRENT_DIR

The environment variable has been removed in rust-lang#14799.
Switch to plain old `CARGO_MANIFEST_DIR`
because snapbox's `current_rs` favors furthest Cargo.toml,
blocking tests running in rust-lang/rust as a submodule.

See also build failures in rust-lang/rust#133533

.cargo/config.toml

@@ -3,8 +3,3 @@ build-man = "run --package xtask-build-man --"
 stale-label = "run --package xtask-stale-label --"
 bump-check = "run --package xtask-bump-check --"
 lint-docs = "run --package xtask-lint-docs --"
-
-[env]
-# HACK: Until this is stabilized, `snapbox`s polyfill could get confused
-# inside of the rust-lang/rust repo because it looks for the furthest-away `Cargo.toml`
-CARGO_RUSTC_CURRENT_DIR = { value = "", relative = true }

src/cargo/util/lints.rs

@@ -610,6 +610,7 @@ mod tests {
     use itertools::Itertools;
     use snapbox::ToDebug;
     use std::collections::HashSet;
+    use std::path::Path;
 
     #[test]
     fn ensure_sorted_lints() {
@@ -647,7 +648,7 @@ mod tests {
 
     #[test]
     fn ensure_updated_lints() {
-        let path = snapbox::utils::current_rs!();
+        let path = Path::new(std::env!("CARGO_MANIFEST_DIR")).join(file!());
         let expected = std::fs::read_to_string(&path).unwrap();
         let expected = expected
             .lines()
@@ -686,7 +687,7 @@ mod tests {
 
     #[test]
     fn ensure_updated_lint_groups() {
-        let path = snapbox::utils::current_rs!();
+        let path = Path::new(std::env!("CARGO_MANIFEST_DIR")).join(file!());
         let expected = std::fs::read_to_string(&path).unwrap();
         let expected = expected
             .lines()