test: Ensure snapbox works until CARGO_RUSTC_CURRENT_DIR is stabilized

Snapbox's polyfill for `CARGO_RUSTC_CURRENT_DIR` looks for the furthest
away `Cargo.toml`.
That works in our repo but won't work as a submodule in rustc.
This uses the `config.toml` hack for setting the variable.

I verified
- Without this, the polyfill is used
- With this, it is used
- Without this on nightly, the official `CARGO_RUSTC_CURRENT_DIR` is
  used

Author: epage

.cargo/config.toml

@@ -2,3 +2,8 @@
 build-man = "run --package xtask-build-man --"
 stale-label = "run --package xtask-stale-label --"
 bump-check = "run --package xtask-bump-check --"
+
+[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 }