fix: ci: don't use hombrew for MacOS (for now)

Reason: it is missing `lake` (see https://github.com/Homebrew/homebrew-core/pull/87486)

.github/workflows/ci.yml

@@ -38,16 +38,18 @@ jobs:
         curl -sSL https://raw.githubusercontent.com/leanprover/elan/master/elan-init.sh > elan-init.sh
         ./elan-init.sh -y
         cygpath -w "$USERPROFILE/.elan/bin" >> $GITHUB_PATH
-    - name: Install Elan (Ubuntu)
-      if: matrix.os == 'ubuntu-latest'
+    - name: Install Elan (Unix)
+      if: matrix.os != 'windows-latest'
+    # - name: Install Elan (Ubuntu)
+    #   if: matrix.os == 'ubuntu-latest'
       run: |
         curl -sSL https://raw.githubusercontent.com/leanprover/elan/master/elan-init.sh > elan-init.sh
         chmod +x elan-init.sh
         ./elan-init.sh -y
         echo "$HOME/.elan/bin" >> $GITHUB_PATH
-    - name: Install Elan (MacOS)
-      if: matrix.os == 'macOS-latest'
-      run: brew install elan
+    # - name: Install Elan (MacOS)
+    #   if: matrix.os == 'macOS-latest'
+    #   run: brew install elan
     - name: Checkout
       uses: actions/checkout@v2
     - name: Check Lean