debian/rules: Use /bin/sh as POSIX shell regardless of build environment

debian/rules: Use /bin/sh as POSIX shell regardless of build environment

While working on the “reproducible builds” effort [1], we have noticed
that xz-utils could not be built reproducibly.

When dash is the default shell, the configure script
m4/posix-shell.m4 will select /bin/bash as the “conforming POSIX
shell”. When bash is the default shell, /bin/sh will be selected.

The binary package currently in sid on amd64 uses /bin/bash. As bash
is currently Essential:yes (#103284), this is probably not a problem.
But I wonder if they would not be troubles if the package was built on
an environment were bash is the default shell, but later installed on a
system where /bin/dash is /bin/sh.

So for reproducibility and safety reason, it would be best if the
selected path to the shell would not depend on the build environment.

[jrnieder@gmail.com: imported from uploaded NMU]