Sanitize :: from path for sytsem shell

Starting in asdf >= 7.0.0 the path would contain :: and cause buildroot ot throw the error:

You seem to have the current working directory in your
PATH environment variable. This doesn't work.
support/dependencies/dependencies.mk:21: recipe for target 'dependencies' failed

lib/mix/nerves/shell.ex

@@ -24,7 +24,8 @@ defmodule Mix.Nerves.Shell do
         :binary,
         :eof,
         :stream,
-        :stderr_to_stdout
+        :stderr_to_stdout,
+        {:env, [{'PATH', sanitize_path()}]}
       ])
 
     # Tell the script command about the terminal dimensions