refactor: replace removed CARGO_RUSTC_CURRENT_DIR

ref: rust-lang/cargo#3946

justfile

@@ -50,6 +50,10 @@ Variables can be set using `just VARIABLE=VALUE ...` or
 
 # env var to set the cargo runner for the riscv64 target
 export CARGO_TARGET_RISCV64GC_K23_NONE_KERNEL_RUNNER := "just _run_riscv64"
+# as of recent Rust nightly versions the old `CARGO_RUSTC_CURRENT_DIR` we used to locate the kernel artifact from the
+# loader build script got removed :/ This is a stopgap until they come up with a replacement.
+# https://github.com/rust-lang/cargo/issues/3946
+export __K23_CARGO_RUSTC_CURRENT_DIR := `dirname "$(cargo locate-project --workspace --message-format plain)"`
 
 # default recipe to display help information
 _default:

loader/build.rs

@@ -2,7 +2,7 @@ use std::path::{Path, PathBuf};
 use std::{env, fs};
 
 fn main() {
-    let workspace_root = Path::new(env!("CARGO_RUSTC_CURRENT_DIR"));
+    let workspace_root = PathBuf::from(env::var_os("__K23_CARGO_RUSTC_CURRENT_DIR").unwrap());
     let out_dir = PathBuf::from(env::var_os("OUT_DIR").unwrap());
 
     println!("cargo::rerun-if-env-changed=KERNEL");