Use package root as cargo check working directory

Cargo commands are affected by the `.cargo/config` files above
their working directory. If cargo is invoked from above the directory
holding `Cargo.toml`, it may not pick up important settings like
registry replacements, causing it to behave differently or even fail.

Most cargo invocations are currently setting their working directories
to the directory containing `Cargo.toml`, but a couple of paths remain
in which cargo is invoked from the default workspace root instead.

This change fixes that, resolving some cargo check failures that I
experienced in a multi-root workspace in which packages used different
registries.

crates/flycheck/src/lib.rs

@@ -215,6 +215,7 @@ impl FlycheckActor {
             } => {
                 let mut cmd = Command::new(toolchain::cargo());
                 cmd.arg(command);
+                cmd.current_dir(&self.workspace_root);
                 cmd.args(&["--workspace", "--message-format=json", "--manifest-path"])
                     .arg(self.workspace_root.join("Cargo.toml"));
 

crates/project_model/src/build_data.rs

@@ -143,6 +143,7 @@ impl WorkspaceBuildData {
             cmd.env("RA_RUSTC_WRAPPER", "1");
         }
 
+        cmd.current_dir(cargo_toml.parent().unwrap());
         cmd.args(&["check", "--quiet", "--workspace", "--message-format=json", "--manifest-path"])
             .arg(cargo_toml.as_ref());