misc_systemd_units/unbound-startup: Explicitly use bash in shebang

The script uses Bashism, so explicitly list Bash as the interpreter, so
the script can also run in operating systems like Ubuntu, where /bin/sh
is Dash.

misc_systemd_units/unbound-startup

@@ -1,4 +1,4 @@
-#! /bin/sh
+#! /bin/bash
 
 #
 # unbound is usuually started  via systemd.