fix(aws.go): load region before attempting to assume a role

[aauren]

@catherinetcai / @mrueg for review

Fixes #2004

[catherinetcai]

Changes look sound to me. Wondering if there's any sort of testing we could invest in to be able to better catch things like this when doing dependency bumps :\

[aauren]

Changes look sound to me. Wondering if there's any sort of testing we could invest in to be able to better catch things like this when doing dependency bumps :\

I looked into this a bit, and there are a few ways to test it, but all of them basically require integration tests to have truly caught something like this. Given how small our surface is to AWS, I think its probably a bit overboard for just now considering this is the first time that I can remember breaking it, and it would add quite a bit over overhead to the testing (probably running something like LocalStack in a container to mimic AWS APIs would be the most portable).