Make install-tools command work from any directory in the workspace. (#2725)

This fixes #2708 by creating a CARGO_WORKSPACE_DIR env variable to act
as an anchor path, allowing the installation of mdbook-exerciser and
mdbook-course to succeed from any directory within the repository. Based
on the approach mentioned here:
rust-lang/cargo#3946 (comment)

---------

Co-authored-by: Eric Githinji <[email protected]>

Author: egithinji

.cargo/config.toml

@@ -2,3 +2,8 @@
 # We use this alias for task automation in the project.
 # See README in xtask directory.
 xtask = "run --package xtask --"
+
+[env]
+# To provide an anchor to the root of the workspace when working with paths.
+# See https://github.com/rust-lang/cargo/issues/3946#issuecomment-973132993
+CARGO_WORKSPACE_DIR = { value = "", relative = true }

xtask/src/main.rs

@@ -21,6 +21,7 @@
 
 use anyhow::{anyhow, Ok, Result};
 use clap::Parser;
+use std::path::Path;
 use std::{env, process::Command};
 
 fn main() -> Result<()> {
@@ -54,6 +55,11 @@ fn execute_task() -> Result<()> {
 fn install_tools() -> Result<()> {
     println!("Installing project tools...");
 
+    let path_to_mdbook_exerciser =
+        Path::new(env!("CARGO_WORKSPACE_DIR")).join("mdbook-exerciser");
+    let path_to_mdbook_course =
+        Path::new(env!("CARGO_WORKSPACE_DIR")).join("mdbook-course");
+
     let install_args = vec![
         // The --locked flag is important for reproducible builds. It also
         // avoids breakage due to skews between mdbook and mdbook-svgbob.
@@ -62,9 +68,11 @@ fn install_tools() -> Result<()> {
         vec!["mdbook-pandoc", "--locked", "--version", "0.9.3"],
         vec!["mdbook-i18n-helpers", "--locked", "--version", "0.3.5"],
         vec!["i18n-report", "--locked", "--version", "0.2.0"],
-        // These packages are located in this repository
-        vec!["--path", "mdbook-exerciser", "--locked"],
-        vec!["--path", "mdbook-course", "--locked"],
+        // Mdbook-exerciser and mdbook-course are located in this repository.
+        // To make it possible to install them from any directory we need to
+        // specify their path from the workspace root.
+        vec!["--path", path_to_mdbook_exerciser.to_str().unwrap(), "--locked"],
+        vec!["--path", path_to_mdbook_course.to_str().unwrap(), "--locked"],
     ];
 
     for args in &install_args {