Specificy explict rust version

By default, rustup installs the "latest" stable version of the rust
toolchain.  Instead, fix this to a specific version.  This serves two
purposes:

- It becomes possible to map a docker image version to a specific
  version of the rust toolchain that it contains.
- It makes upgrades explicit, especially when features of a specific
  version are needed.

This sets the version to the current stable version: 1.86.

Signed-off-by: David Brown <david.brown@linaro.org>

Author: d3zd3z

Dockerfile.ci

@@ -88,7 +88,7 @@ RUN mkdir -p /opt/bsim_west && \
 	ln -s /opt/bsim_west/bsim /opt/bsim
 
 # Install cargo environment
-RUN wget -q -O-  "https://sh.rustup.rs" | sh -s -- -y
+RUN wget -q -O-  "https://sh.rustup.rs" | sh -s -- -y --default-toolchain 1.86
 
 # Install uefi-run utility
 RUN ~/.cargo/bin/cargo install uefi-run --root /usr