build: Set default toolchain

Author: lenianiva

.github/workflows/test.yaml

@@ -21,7 +21,7 @@ jobs:
       run: |
         set -o pipefail
         curl -sSfL https://github.com/leanprover/elan/releases/download/v3.1.1/elan-x86_64-unknown-linux-gnu.tar.gz | tar xz
-        ./elan-init -y --default-toolchain none
+        ./elan-init -y --default-toolchain $(<src/lean-toolchain)
         echo "$HOME/.elan/bin" >> "${GITHUB_PATH}"
 
     - name: Install Lean
@@ -31,6 +31,7 @@ jobs:
     - name: Install uv
       run: |
         curl -LsSf https://astral.sh/uv/install.sh | sh
+        elan default $(<src/lean-toolchain)
         uv sync
 
     - name: Run tests