Add a default PATH variable to OS scripts env

In commit 896a03f6 I cleaned up the environment for OS scripts,
however I think that was a bit too extreme - it breaks our own
instance-debootstrap hooks, because for example dpkg (called from the
grub script) requires PATH to be set.

Instead of requiring every OS to define a path, let's set a default
PATH for the OS scripts, which should cover most common uses. A more
specialised PATH can be set, if needed, in the OS scripts.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>

lib/backend.py

@@ -2204,6 +2204,11 @@ def OSCoreEnv(os_name, inst_os, os_params, debug=0):
   for pname, pvalue in os_params.items():
     result["OSP_%s" % pname.upper()] = pvalue
 
+  # Set a default path otherwise programs called by OS scripts (or
+  # even hooks called from OS scripts) might break, and we don't want
+  # to have each script require setting a PATH variable
+  result["PATH"] = constants.HOOKS_PATH
+
   return result