xtask: Fix channel of cargo operations

It seems that some recent change (in cargo? or maybe rustc or rustup)
broke the way we run cargo from xtask. The channel arg (e.g. "+nightly")
is getting ignored due to the RUSTC env var and other similar vars,
which contain full paths to a particular channel's binaries.

Fix by unsetting those variables in the child cargo's env.

Author: nicholasbishop

xtask/src/cargo.rs

@@ -101,6 +101,12 @@ impl Cargo {
     pub fn command(&self) -> Result<Command> {
         let mut cmd = Command::new("cargo");
 
+        // Cargo automatically sets some env vars that can prevent the
+        // channel arg (e.g. "+nightly") from working. Unset them in the
+        // child's environment.
+        cmd.env_remove("RUSTC");
+        cmd.env_remove("RUSTDOC");
+
         if let Some(toolchain) = &self.toolchain {
             cmd.arg(&format!("+{}", toolchain));
         }
@@ -206,7 +212,7 @@ mod tests {
         };
         assert_eq!(
             command_to_string(&cargo.command().unwrap()),
-            "RUSTDOCFLAGS=-Dwarnings cargo +nightly doc --package uefi --package xtask --features alloc --open"
+            "RUSTC= RUSTDOC= RUSTDOCFLAGS=-Dwarnings cargo +nightly doc --package uefi --package xtask --features alloc --open"
         );
     }
 }

xtask/src/main.rs

@@ -173,13 +173,15 @@ fn test_latest_release() -> Result<()> {
     // it explicit that it matches the command in `BUILDING.md`.
     let mut build_cmd = Command::new("cargo");
     build_cmd
+        .env_remove("RUSTC")
         .args(&["+nightly", "build", "--target", "x86_64-unknown-uefi"])
         .current_dir(tmp_dir.join("template"));
 
     // Check that the command is indeed in BUILDING.md, then verify the
     // build succeeds.
     let building_md = include_str!("../../BUILDING.md");
-    assert!(building_md.contains(&command_to_string(&build_cmd)));
+    let cmd_str = command_to_string(&build_cmd).replace("RUSTC= ", "");
+    assert!(building_md.contains(&cmd_str));
     run_cmd(build_cmd)
 }