use bootstrap toolchain

Author: pitaj

.github/workflows/dependencies.yml

@@ -17,6 +17,8 @@ env:
   CARGO_UPGRADES_VER: --version=1.3.6
   PR_TITLE: Weekly `cargo update`
   PR_MESSAGE: |
+    Automation to keep dependencies in `Cargo.lock` current.
+
     In addition to the semver-compatible updates in this PR,
     the following major upgrades are also available:
 
@@ -28,12 +30,14 @@ jobs:
       - name: checkout the source code
         uses: actions/checkout@v3
         with:
-          fetch-depth: 2
           submodules: recursive
-      - name: install the latest nightly
+      - name: install the bootstrap toolchain
         run: |
-          rustup toolchain install nightly
-          rustup default nightly
+          # Extract the stage0 version
+          TOOLCHAIN=$(jq -r '.compiler | {version,date} | join("-")' -- src/stage0.json)
+          # Install and set as default
+          rustup toolchain install $TOOLCHAIN
+          rustup default $TOOLCHAIN
       - name: install cargo-upgrades
         run: cargo install -f cargo-upgrades $CARGO_UPGRADES_VER