build: add opam update to static-build.sh

Otherwise, opam is unable to find a solution to the dependencies of
learn-ocaml with the new pinned package versions.

Author: tsani

scripts/static-build.sh

@@ -14,7 +14,8 @@ git ls-files -z | xargs -0 tar c | \
     sh -uexc \
       'tar x >&2 &&
        sudo apk add openssl-libs-static bash >&2 &&
-       opam switch create . ocaml-system --deps-only --locked >&2 &&
+       opam update >&2 &&
+       opam switch create . 5.1.1 --deps-only --locked >&2 &&
        opam exec make LINKING_MODE=static >&2 &&
        tar c -hC _build/install/default/bin .' | \
   tar vx