Remove the need for rust nightly

My pidfd implmentation is simple enough, and Option.contains() is not
worth requiring nightly.

Dockerfile

@@ -9,7 +9,7 @@ RUN apt-get update && \
     apt-get -y install curl devscripts && \
     mkdir -p /build
 
-RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain nightly
+RUN curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
 
 # Do this first to get our build deps cached.
 COPY debian ./debian