CI: install lambdapi outside of hol2dk repository

Author: agontard

.github/workflows/main.yml

@@ -36,10 +36,9 @@ jobs:
         run: opam install -y dune.${{ matrix.dune-version }}
       - name: Install lambdapi
         run: |
-          git clone --depth 1 -b ${{ matrix.lambdapi-version }} https://github.com/Deducteam/lambdapi
+          git clone --depth 1 -b ${{ matrix.lambdapi-version }} https://github.com/Deducteam/lambdapi ../lambdapi
           sudo apt-get install -y libev-dev
-          opam pin lambdapi lambdapi
-          opam install -y lambdapi
+          opam pin lambdapi ../lambdapi
       - name: Compile hol2dk
         run: |
           eval `opam env`