chore: bump toolchain to v4.26.0-rc2 (#146)

* chore: bump toolchain to v4.26.0-rc2

* fix: use setup-node to ensure consistent npm version in CI

The previous approach of running `npm install npm` on Windows installed
npm as a local package instead of updating the system npm. This caused
lockfile sync errors when `npm ci` ran later in the build.

Using setup-node@v4 with Node 20 ensures all platforms use a consistent,
modern npm version that's compatible with the package-lock.json format.

Author: kim-em

.github/workflows/build.yml

@@ -40,11 +40,10 @@ jobs:
 
       - uses: actions/checkout@v4
 
-      - name: Update NPM (Windows)
-        shell: bash
-        if: matrix.os == 'windows-latest'
-        # See https://github.com/nodejs/node/issues/52682
-        run: cd widget/ && npm install npm
+      - name: Setup Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: '20'
 
       - name: Build package
         run: lake build ProofWidgets

lean-toolchain

@@ -1 +1 @@
-leanprover/lean4:v4.26.0-rc1
+leanprover/lean4:v4.26.0-rc2