Consistently use same Rust nightly across CI and Docker images

This should fix the nightly `crux-mir` Docker job. While I was in town, I made
sure to leave breadcrumbs to the reader such that if they ever update the Rust
nightly in one spot, they will remember to update it in the other spot.

.github/Dockerfile-crux-mir

@@ -1,13 +1,19 @@
+# If you update this, make sure to also update RUST_TOOLCHAIN in
+# .github/workflows/crux-mir-build.yml
+ARG RUST_TOOLCHAIN="nightly-2023-01-23"
+
 FROM rust:1.46.0 AS mir_json
+ARG RUST_TOOLCHAIN
 
 ADD dependencies/mir-json /mir-json
 WORKDIR /mir-json
-RUN rustup toolchain install nightly-2020-03-22 --force
-RUN rustup component add --toolchain nightly-2020-03-22 rustc-dev
-RUN rustup default nightly-2020-03-22
+RUN rustup toolchain install ${RUST_TOOLCHAIN} --force
+RUN rustup component add --toolchain ${RUST_TOOLCHAIN} rustc-dev
+RUN rustup default ${RUST_TOOLCHAIN}
 RUN cargo install --locked
 
 FROM ubuntu:22.04 AS build
+ARG RUST_TOOLCHAIN
 
 RUN apt-get update && \
     apt-get install -y \
@@ -58,7 +64,7 @@ RUN cabal v2-build --only-dependencies crux-mir
 RUN cabal v2-build crux-mir
 RUN cp `cabal v2-exec which crux-mir` /usr/local/bin
 RUN cp /usr/local/cargo/bin/* /usr/local/bin/
-RUN rustup default nightly-2020-03-22
+RUN rustup default ${RUST_TOOLCHAIN}
 WORKDIR ${DIR}/build/crux-mir
 RUN ./translate_libs.sh
 WORKDIR ${DIR}/build

.github/workflows/crux-mir-build.yml

@@ -20,6 +20,9 @@ env:
   # (symptom: "No suitable image found ... unknown file type, first
   # eight bytes: 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00")
   CACHE_VERSION: 4
+  # If you update this, make sure to also update RUST_TOOLCHAIN in
+  # .github/Dockerfile-crux-mir
+  RUST_TOOLCHAIN: "nightly-2023-01-23"
 
 jobs:
   config:
@@ -101,7 +104,7 @@ jobs:
       - name: Install latest Rust nightly
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly-2023-01-23
+          toolchain: ${{ env.RUST_TOOLCHAIN }}
           override: true
           components: rustc-dev