CI: Set and update weekly the stable toolchain version

Currently we are using `stable` for the stable toolchain. This is ok
but it means we get whatever the Rust project pushes. Instead lets be
explicit so we know exactly what version we get. Also add a cron job
to update it weekly.

Author: tcharding

.github/workflows/cron-weekly-update-stable.yml

@@ -0,0 +1,35 @@
+name: Update Stable rustc
+on:
+  schedule:
+    - cron: "0 0 * * 5" # runs every Friday at 00:00 (generally rust releases on Thursday)
+  workflow_dispatch: # allows manual triggering
+permissions: {}
+jobs:
+  format:
+    name: Update stable rustc
+    runs-on: ubuntu-24.04
+    permissions:
+      contents: write
+      id-token: write
+      pull-requests: write
+    steps:
+      - uses: actions/checkout@08c6903cd8c0fde910a37f88322edcfb5dd907a8 # v5.0.0
+        with:
+          persist-credentials: false
+      - name: Install cargo-rbmt
+        run: cargo install --git https://github.com/rust-bitcoin/rust-bitcoin-maintainer-tools.git --rev $(cat rbmt-version) cargo-rbmt
+      - name: Update stable toolchain in Cargo.toml
+        run: |
+          eval "$(cargo rbmt toolchains --update-stable)"
+          echo "stable_version=$RBMT_STABLE" >> $GITHUB_ENV
+      - name: Create Pull Request
+        uses: peter-evans/create-pull-request@271a8d0340265f705b14b6d32b9829c1cb33d45e # v7.0.8
+        with:
+          token: ${{ secrets.APOELSTRA_RUST_BITCOIN_ORG_CREATE_PR }}
+          author: Update Stable Rustc Bot <bot@example.com>
+          committer: Update Stable Rustc Bot <bot@example.com>
+          title: Automated weekly update to rustc stable (to ${{ env.stable_version }})
+          body: |
+           Automated update to Cargo.toml workspace metadata by [create-pull-request](https://github.com/peter-evans/create-pull-request) GitHub action
+          commit-message: Automated update to rustc stable-${{ env.stable_version }}
+          branch: create-pull-request/weekly-stable-update

Cargo.toml

@@ -14,7 +14,7 @@ rust-version = "1.56.1"
 
 [package.metadata.rbmt.toolchains]
 nightly = "nightly-2026-06-05"
-stable = "stable"
+stable = "1.95.0"
 
 [features]
 default = ["std"]