Patch setting of AWS credential file environment variable

[salvis2]

@scottyhq I see that you changed the CircleCI action to move credentials to the standard location because the environment variable didn't work: https://github.com/pangeo-data/pangeo-binder/commit/7cf3c2bf7febabb9981597e082fa676f8b5c12b9#diff-78a8a19706dbd2a4425dd72bdab0502ed7a2cef16365ab7030a5a0588927bf47

I think you need to put export at the beginning of those commands. I fixed this on the hubs in https://github.com/pangeo-data/pangeo-cloud-federation/commit/cede57e5215f4d29792efe53b66a865dd5b56ab4#diff-0996279a7b3793d02cc5d648b6034c09effa4e6d2d3e6c82dc1f57179bb0e45fR47.