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>