Bash not found on some distros

[aerique]

Both build.sh and gen-version use #!/bin/bash instead of #!/usr/bin/env bash which means Bash will not always be found (even on some Linux distributions!).

• https://github.com/PowerDNS/pdns-builder/blob/master/build.sh#L1
• https://github.com/PowerDNS/pdns-builder/blob/master/gen-version#L1