Handle non-script grub2-install

grub2-install has been re-written [1] so isn't always a shell script,
so just exec it directly.  Also use $() as that follows the rest of
the file.

[1] http://git.savannah.gnu.org/cgit/grub.git/commit?id=cd46aa6cefabd7b0fcb15cd614577e1ab8c7a841

Change-Id: Ib4fd7c6854fb485c0da6a79babf4a3d45a8a0f2c
Merge "Handle non-script grub2-install"