gha: pin sha.1.15.4 on Windows for fixes

Author: MisterDA

.github/workflows/main.yml

@@ -121,6 +121,11 @@ jobs:
         uses: ocaml/setup-ocaml@v2
         with:
           ocaml-compiler: ${{ matrix.ocaml-compiler }}
+          opam-depext: false
+
+      - run: |
+          opam pin -yn sha.1.15.4 'git+https://github.com/djs55/ocaml-sha.git#c743398abee8f822fc0d12f229121e431d60dd5d'
+          D:\cygwin\wrapperbin\opam.cmd depext obuilder-spec obuilder
 
       - run: opam install . --deps-only --with-test