virtiofsd: Pass the expected toolchain to the build container

Let's ensure we're building virtiofsd with a specific toolchain that's
known to not cause any issues, instead of always using the latest one.

On each bump of the virtiofsd, we'll make sure to adjust this according
to what's been used by the virtiofsd community.

Signed-off-by: Fabiano Fidêncio <fabiano.fidencio@intel.com>

tools/packaging/static-build/virtiofsd/gnu/Dockerfile

@@ -4,6 +4,7 @@
 
 FROM ubuntu:20.04
 ENV DEBIAN_FRONTEND=noninteractive
+ARG RUST_TOOLCHAIN
 SHELL ["/bin/bash", "-o", "pipefail", "-c"]
 
 RUN apt-get update && \
@@ -16,4 +17,4 @@ RUN apt-get update && \
         libseccomp-dev \
         unzip && \
     apt-get clean && rm -rf /var/lib/lists/ && \
-    curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y
+    curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain ${RUST_TOOLCHAIN}