[3844] Use rootpath for wrapper root for determinism (#3845)

Fixes #3844

Uses a relative path instead of absolute:
```
WRAPPED_TOOL_EXECPATH: ../+rust+rust_linux_x86_64__x86_64-unknown-linux-gnu__stable_tools/rust_toolchain/bin/cargo
```

---------

Co-authored-by: Daniel Wagner-Hall <dawagner@gmail.com>

Author: finn-ball

tools/upstream_wrapper/BUILD.bazel

@@ -21,8 +21,8 @@ ALL_TOOLS = [target for target in TOOLS.values()]
         data = ALL_TOOLS if tool_name == "cargo" else [target],
         edition = "2018",
         rustc_env = {
-            "WRAPPED_TOOL_EXECPATH": "$(execpath {})".format(target),
             "WRAPPED_TOOL_NAME": tool_name,
+            "WRAPPED_TOOL_ROOTPATH": "$(rootpath {})".format(target),
             "WRAPPED_TOOL_TARGET": "$(rlocationpath {})".format(target),
         },
         toolchains = ["//rust/toolchain:current_rust_toolchain"],

tools/upstream_wrapper/src/main.rs

@@ -4,7 +4,7 @@ use std::process::{exit, Command};
 
 const WRAPPED_TOOL_NAME: &str = env!("WRAPPED_TOOL_NAME");
 const WRAPPED_TOOL_TARGET: &str = env!("WRAPPED_TOOL_TARGET");
-const WRAPPED_TOOL_EXECPATH: &str = env!("WRAPPED_TOOL_EXECPATH");
+const WRAPPED_TOOL_ROOTPATH: &str = env!("WRAPPED_TOOL_ROOTPATH");
 
 #[cfg(not(target_os = "windows"))]
 const PATH_SEPARATOR: &str = ":";
@@ -20,7 +20,11 @@ fn main() {
             }
             Ok(path)
         })
-        .unwrap_or(PathBuf::from(WRAPPED_TOOL_EXECPATH));
+        .unwrap_or_else(|_| {
+            std::env::current_dir()
+                .expect("Failed to get current directory")
+                .join(WRAPPED_TOOL_ROOTPATH)
+        });
 
     if !wrapped_tool_path.exists() {
         panic!(
@@ -39,7 +43,7 @@ fn main() {
 
     let working_directory = std::env::var_os("BUILD_WORKING_DIRECTORY")
         .map(PathBuf::from)
-        .unwrap_or_else(|| std::env::current_dir().expect("Failed to get working directory"));
+        .unwrap_or_else(|| std::env::current_dir().expect("Failed to get build working directory"));
 
     let mut command = Command::new(wrapped_tool_path);
     command