grub2: only add speech synth install entry if missing

live-build commit 24aab15d2a133075992ce3a216b1c3476dc7bd4c has added
generation of speech synth entries. this ensures that with the next
release of live-build, the kali one is not added as a duplicate.

we have to take a different approach here than for syslinux. the approach
relies upon an expectation that in the next live-build release the grub
config will include an install.cfg file containing install entries, like
syslinux, as a result of changes awaiting merge in MR #135 ([1]).

[1]: https://salsa.debian.org/live-team/live-build/-/merge_requests/135

kali-config/common/bootloaders/grub-pc/grub.cfg

@@ -46,10 +46,12 @@ menuentry "Live system (encrypted persistence, check kali.org/prst)" {
 # Installer (if any)
 LINUX_INSTALL
 
+if [ ! -e /boot/grub/install.cfg ]; then
 menuentry "Start installer with speech synthesis" {
 	linux KERNEL_GI speakup.synth=soft APPEND_GI 
 	initrd INITRD_GI
 }
+fi
 
 submenu 'Advanced options...' {