Update auto demo docs to mention environment

Explicitly state the legacy toolchain environment required to build the
automotive demo's legacy component, and add example commands that show
entering and exiting that environment to build the component. Also point
out that this is only available on Linux systems.

examples/automotive/README.md

@@ -39,9 +39,16 @@ Ethernet cable.
 ## Building
 
 The cheriot componenents of this demo are built along with the rest of the examples.
-However, the legacy component needs to be build seperately.
+However, the legacy component needs to be built seperately. The legacy component
+requires the legacy RISC-V toolchain that is available on Linux systems in the
+`build-legacy-software` Nix environment. So, to build the full demo, you can run:
 
 ```sh
+# In the normal `nix develop` environment.
 xmake -P examples
+# Enter an environment with the legacy toolchain, and build the legacy component.
+nix develop .#build-legacy-software
 xmake -P examples/automotive/legacy/
+# Exit the overlaid legacy environment
+exit
 ```