discover/grub2: Do not set $0 in functions

GRUB2 does not set $0 in functions, so don't set it in our GRUB2
script parser.  (As it doesn't have a value in GRUB2 scripts, probably
no GRUB2 script depends on the value of $0.)  Additionally, dash and
bash set environment variable 0 to the name of the script (even in
functions), so the current behavior of $0 doesn't really match shell
scripts either.

Tested:
 Existing tests pass.

Signed-off-by: Alan Dunn <amdunn@google.com>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>