plymouth-set-default-theme: Remove bashisms

This is run in an environment without bash. Move it to /bin/sh and remove any bashism.

Plymouth post installation is failing to run the following script as it's a bash script.

/usr/share/initramfs-tools/hooks/plymouth: 21: /usr/sbin/plymouth-set-default-theme: not found

Thus, debian/local/plymouth.hook exits early, without copying binaries to initrd disk.