fixup! Hackish workaround for ocurrent/obuilder#77

lib/opam_build.ml

@@ -37,12 +37,13 @@ let opam_install ~variant ~opam_version ~pin ~lower_bounds ~with_tests ~pkg =
   (if with_tests then [
      (* TODO: Remove this hack when https://github.com/ocurrent/obuilder/issues/77 is fixed *)
      (* NOTE: This hack will fail for packages that have src: "git+https://..." *)
-     run ~cache ~network "(%sopam reinstall --with-test %s) || true"
+     run ~network "(%sopam reinstall --with-test %s) || true"
        (match opam_version with `V2_1 | `Dev -> "" | `V2_0 -> fmt "opam depext%s %s && " with_tests_opt pkg) pkg
    ] else []) @ [
     (* TODO: Replace by two calls to opam install + opam install -t using the OPAMDROPINSTALLEDPACKAGES feature *)
     (* NOTE: See above for the ~network:(if with_tests ...) hack *)
-    run ~cache ~network:(if with_tests then [] else network)
+    (* NOTE: We cannot use the cache as concurrent access to the cache might overwrite it and the required archives might not be available anymore at this point *)
+    run ~cache:(if with_tests then [] else cache) ~network:(if with_tests then [] else network)
       {|%sopam reinstall%s%s %s;
         res=$?;
         test "$res" != 31 && exit "$res";