rootfs: Install Rust only when necessary

For docker-based builds only install Rust when necessary.
Further, execute the detect Rust version check only when
intending to install Rust.
As of today, this is the case when we intend to build the
agent during rootfs build.

Signed-off-by: Manuel Huber <mahuber@microsoft.com>

tools/osbuilder/rootfs-builder/rootfs.sh

@@ -385,12 +385,6 @@ build_rootfs_distro()
 		mkdir -p ${ROOTFS_DIR}
 	fi
 
-	# need to detect rustc's version too?
-	detect_rust_version ||
-		die "Could not detect the required rust version for AGENT_VERSION='${AGENT_VERSION:-main}'."
-
-	echo "Required rust version: $RUST_VERSION"
-
 	if [ "${SELINUX}" == "yes" ]; then
 		if [ "${AGENT_INIT}" == "yes" ]; then
 			die "Guest SELinux with the agent init is not supported yet"

tools/osbuilder/scripts/lib.sh

@@ -225,14 +225,23 @@ generate_dockerfile()
 
 	[ -n "${http_proxy:-}" ] && readonly set_proxy="RUN sed -i '$ a proxy="${http_proxy:-}"' /etc/dnf/dnf.conf /etc/yum.conf; true"
 
-	# Rust agent
-	readonly install_rust="
+	# Only install Rust if agent needs to be built
+	local install_rust=""
+
+	if [ ! -z "${AGENT_SOURCE_BIN}" ] ; then
+		if [ "$RUST_VERSION" == "null" ]; then
+			detect_rust_version || \
+				die "Could not detect the required rust version for AGENT_VERSION='${AGENT_VERSION:-main}'."
+		fi
+		install_rust="
 ENV http_proxy=${http_proxy:-}
 ENV https_proxy=${http_proxy:-}
 RUN curl --proto '=https' --tlsv1.2 https://sh.rustup.rs -sSLf | \
     sh -s -- -y --default-toolchain ${RUST_VERSION} -t ${rustarch}-unknown-linux-${LIBC}
 RUN . /root/.cargo/env; cargo install cargo-when
 "
+	fi
+
 	pushd "${dir}"
 
 	sed \