If toolchain cannot be found, point to `available` command.

mpsd-software.py

@@ -885,8 +885,8 @@ def install_environment(
 
     for toolchain in toolchains:
         if toolchain not in available_toolchains:
-            # TODO: add to message how toolchains can be found
             msg = f"Toolchain '{toolchain}' is not available in release {mpsd_release}."
+            msg += f"Use 'available' command to see list of available toolchains."
             logging.error(msg)
             sys.exit(1)