install.sh: Change interpreter to /bin/bash

/bin/sh is the same as /usr/bin/bash on mariux, but not on
flughafenberlinbrandenburgwillybrandt. Because we use bash syntax, be
explicit here are ask for bash.

install.sh

@@ -1,4 +1,4 @@
-#! /bin/sh
+#! /bin/bash
 
 # Although these should be inherited from the Makefile,
 # we set some defaults ourself, too, in case we are called