Switch default PAM detection strategy to target-based. To make cross-compilation easier (like for docs.rs), this change makes OS-based detection of PAM the default, only falling back to probing the actual installed PAM as a last resort. I haven't been able to find a Linux distribution that uses anything but Linux-PAM.

FROM docker.io/debian:stable

RUN apt-get -y update \
    && apt-get -y upgrade \
    && apt-get install -y cargo mercurial \
    && apt-get clean