[release] skip opam switch setup in release script when already configured

Summary:
The release script unconditionally ran `build-infer.sh --only-setup-opam`
which created a fresh opam switch, losing the LLVM OCaml bindings
installed during the prior build step. This caused `ctypes-foreign` to
fail during `make install-with-libs` on macOS with a path error.

Only set up the opam switch when one is not already active.

Reviewed By: ngorogiannis

Differential Revision: D103987909

fbshipit-source-id: ba642637ba3f9432cbc67087d1e88ebb1c955787

Author: davidpichardie

scripts/create_binary_release.sh

@@ -38,7 +38,9 @@ JOBS=${JOBS:-$NCPUS}
 pushd "$ROOT_DIR"
 rm -fr "$RELEASE_NAME"
 
-./build-infer.sh --only-setup-opam
+if ! opam switch show >/dev/null 2>&1; then
+  ./build-infer.sh --only-setup-opam
+fi
 eval $(opam env)
 touch .release
 ./autogen.sh