Prep work for overriding sysroot

The idea here is to make auto-discovery optional, and to allow to set
the path to sysroot directly. This is handy for JSON projects.

crates/project_model/src/lib.rs

@@ -561,5 +561,5 @@ fn utf8_stdout(mut cmd: Command) -> Result<String> {
         }
     }
     let stdout = String::from_utf8(output.stdout)?;
-    Ok(stdout)
+    Ok(stdout.trim().to_string())
 }