Adds back sed command for grub

Adding the old style sed command instead of the perl command.
This makes sure that we have only one value within double quotes
in the correct location in /etc/default/grub.

install_files/securedrop-grsec-focal/DEBIAN/postinst.j2

@@ -28,7 +28,7 @@ set_grub_default() {
     # When using CONFIG_PAX_KERNEXEC, the grsecurity team recommends the kernel
     # is booted with "noefi" on the kernel command line if "CONFIG_EFI" is
     # enabled, as EFI runtime services are necessarily mapped as RWX.
-    perl -pi -e 's|^GRUB_CMDLINE_LINUX_DEFAULT=|GRUB_CMDLINE_LINUX_DEFAULT="noefi ipv6.disable=1"|' /etc/default/grub
+    sed -i '/^GRUB_CMDLINE_LINUX_DEFAULT=/s/=.*/=\"noefi ipv6\.disable=1 quiet\"/' /etc/default/grub
     update-grub
 }