Use dev version of sail from github for CI.

The github version of sail has some fixes we need for latex output.
Let's pin CI to this commit until a new sail is released.

.github/workflows/compile.yml

@@ -13,7 +13,7 @@ jobs:
     - name: Init opam
       run: opam init -y
     - name: Install sail
-      run: opam pin -y sail 0.17.1
+      run: git clone -n https://github.com/rems-project/sail.git && cd sail && git checkout 6be215fc13678b4c9c6b0a1d097121da9099c1cb && opam install -y .
     - name: Check out repository code
       uses: actions/checkout@HEAD
       with: