Revert "ci: Pin rustc to version 1.78 due to a type inference regression"

This reverts commit 76ad48cd1b.

.github/scripts/setup.sh

@@ -1,10 +1,7 @@
 #!/bin/bash
 set -e
 export DEBIAN_FRONTEND=noninteractive
-
-# Fix the RUST_VERSION as 1.80 appears to have a regression (the
-# `time` crate about a missing type annotation.)
-export RUST_VERSION=1.78
+export RUST_VERSION=stable
 
 sudo useradd -ms /bin/bash tester
 sudo apt-get update -qq
@@ -59,9 +56,6 @@ sudo chmod 0440 /etc/sudoers.d/tester
 curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- \
      -y --default-toolchain ${RUST_VERSION}
 
-echo "rustc version = $(rustc --version)"
-echo "cargo version = $(cargo --version)"
-
 # We also need a relatively recent protobuf-compiler, at least 3.12.0,
 # in order to support the experimental `optional` flag.