ci: pin nightly compiler version

Don't bother with the cronjob to update this. We'll just leave the nightly
compiler version fixed forever on these old versions (or update it on an
ad-hoc basis).

Author: apoelstra

.github/workflows/rust.yml

@@ -9,6 +9,17 @@ on:   # yamllint disable-line rule:truthy
 name: Continuous integration
 
 jobs:
+  Prepare:
+    runs-on: ubuntu-24.04
+    outputs:
+      nightly_version: ${{ steps.read_toolchain.outputs.nightly_version }}
+    steps:
+      - name: "Checkout repo"
+        uses: actions/checkout@v4
+      - name: "Read nightly version"
+        id: read_toolchain
+        run: echo "nightly_version=$(cat nightly-version)" >> $GITHUB_OUTPUT
+
   Stable:                       # 2 jobs, one per lock file.
     name: Test - stable toolchain
     runs-on: ubuntu-latest
@@ -33,6 +44,7 @@ jobs:
 
   Nightly:                      # 2 jobs, one per lock file.
     name: Test - nightly toolchain
+    needs: Prepare
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
@@ -47,7 +59,9 @@ jobs:
           repository: rust-bitcoin/rust-bitcoin-maintainer-tools
           path: maintainer-tools
       - name: "Select toolchain"
-        uses: dtolnay/rust-toolchain@nightly
+        uses: dtolnay/rust-toolchain@v1
+        with:
+          toolchain: ${{ needs.Prepare.outputs.nightly_version }}
       - name: "Set dependencies"
         run: cp Cargo-${{ matrix.dep }}.lock Cargo.lock
       - name: "Run test script"
@@ -79,6 +93,7 @@ jobs:
 
   Lint:
     name: Lint - nightly toolchain
+    needs: Prepare
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
@@ -93,7 +108,9 @@ jobs:
           repository: rust-bitcoin/rust-bitcoin-maintainer-tools
           path: maintainer-tools
       - name: "Select toolchain"
-        uses: dtolnay/rust-toolchain@nightly
+        uses: dtolnay/rust-toolchain@v1
+        with:
+          toolchain: ${{ needs.Prepare.outputs.nightly_version }}
       - name: "Install clippy"
         run: rustup component add clippy
       - name: "Set dependencies"
@@ -125,6 +142,7 @@ jobs:
 
   Docsrs:
     name: Docs - nightly toolchain
+    needs: Prepare
     runs-on: ubuntu-latest
     strategy:
       fail-fast: false
@@ -139,7 +157,9 @@ jobs:
           repository: rust-bitcoin/rust-bitcoin-maintainer-tools
           path: maintainer-tools
       - name: "Select toolchain"
-        uses: dtolnay/rust-toolchain@nightly
+        uses: dtolnay/rust-toolchain@v1
+        with:
+          toolchain: ${{ needs.Prepare.outputs.nightly_version }}
       - name: "Set dependencies"
         run: cp Cargo-${{ matrix.dep }}.lock Cargo.lock
       - name: "Run test script"
@@ -198,6 +218,7 @@ jobs:
         run: ./contrib/integration_test.sh
 
   Embedded:
+    needs: Prepare
     runs-on: ubuntu-latest
     steps:
       - name: Checkout
@@ -208,7 +229,7 @@ jobs:
         uses: actions-rs/toolchain@v1
         with:
           profile: minimal
-          toolchain: nightly
+          toolchain: ${{ needs.Prepare.outputs.nightly_version }}
           override: true
           components: rust-src
           target: thumbv7m-none-eabi

nightly-version

@@ -0,0 +1 @@
+nightly-2025-03-21