bootstrap.sh: use /usr/bin/env to find bash

bootstrap.sh: use /usr/bin/env to find bash

bootstrap.sh uses some non-POSIX features of bash, so we can't use
/bin/sh.  Unlike /bin/sh, bash can be installed anywhere in the path, so
we should use /usr/bin/env to find it.

This helps systems that have bash in /usr/local/bin, for example.