CI: use ghcr docker image + simplify script

.github/workflows/ci.yml

@@ -17,30 +17,20 @@ jobs:
     name: Check Hakyber EasyCrypt Project
     runs-on: ubuntu-20.04
     container:
-      image: easycryptpa/ec-build-box
+      image: ghcr.io/easycrypt/ec-build-box
     strategy:
       fail-fast: false
     steps:
     - uses: actions/checkout@v4
       with:
         submodules: recursive
-    - uses: actions/checkout@v4
-      name: Checkout EasyCrypt
-      with:
-        repository: EasyCrypt/easycrypt
-        ref: refs/heads/main
-        path: easycrypt
-    - name: Update OPAM & EasyCrypt dependencies
-      run: |
-        opam update
-        opam pin alt-ergo 2.5.2
-        opam pin add -n easycrypt easycrypt
-        opam install --deps-only easycrypt
     - name: Compile & Install EasyCrypt
-      run: opam install easycrypt
+      run: |
+        opam pin --dev-repo add -n easycrypt https://github.com/EasyCrypt/easycrypt.git
+        opam install -v easycrypt
     - name: Detect SMT provers
       run: |
-        rm -f ~/.why3.conf
-        opam config exec -- easycrypt why3config -why3 ~/.why3.conf
+        opam exec -- easycrypt why3config
     - name: Compile Project
-      run: opam config exec -- make checkec
+      run: |
+        opam exec -- make checkec