RUST-1673 Pin Rust version in CI to 1.69 (#887)

Author: isabelatkinson

.evergreen/install-dependencies.sh

@@ -27,6 +27,8 @@ for arg; do
 
         source .evergreen/configure-rust.sh
         rustup toolchain install nightly -c rustfmt
+        # TODO RUST-1674: remove this workaround
+        rustup default 1.69
     elif [ $arg == "mdbook" ]; then
         source ${CARGO_HOME}/env
         # Install the manual rendering tool