[opam] workaround opam ignoring the lock file with --locked

Summary:
This seems like a bug in opam, probably started after we moved the opam
files to opam/ in D27326495 (673c944). I reported the strange behaviour in
ocaml/opam#4313 (comment).

Reviewed By: rgrig

Differential Revision: D28837624

fbshipit-source-id: f91a58dbc

build-infer.sh

@@ -143,9 +143,9 @@ setup_opam () {
 install_opam_deps () {
     local locked=
     if [ "$USE_OPAM_LOCK" == yes ]; then
-        locked=--locked
+        locked=.locked
     fi
-    opam install --deps-only "$INFER_ROOT"/opam/infer.opam $locked &&
+    opam install --deps-only "$INFER_ROOT"/opam/infer.opam$locked &&
     if [ -n "$SANDCASTLE" ]; then
         opam pin list | grep yojson || opam pin add yojson "${DEPENDENCIES_DIR}/yojson-1.7.0fix"
     fi