[PR] [circle] set OPAMDOWNLOADJOBS=1 on Windows
[PR] [circle] set OPAMDOWNLOADJOBS=1 on Windows

Summary:
when multiple opam packages have the same source, like `dune` and `dune-configurator`, and opam downloads them in parallel, there's a race in which they both try to write the same file to the cache. it seems that on Windows, the `cp` fails with `permission denied`, while on other platforms the `cp` just overwrites. by turning off parallel downloads, opam skips downloading the duplicate source because it's already cached.

See fdopen/opam-repository-mingw#71
Pull Request resolved: https://github.com/facebook/flow/pull/8268

Differential Revision: D19449255

Pulled By: mroch

fbshipit-source-id: a695c160026d523fb59412793f49fc74ec7d112e

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow