fix: pass creds via env vars if vercel

Summary:
fix vercel build https://vercel.com/elementl/dagster/CRmVHxrc72M4p3BfD8iwW3XEXeMq
the aws creds weren't correctly configured in the vercel envs. according to https://vercel.com/support/articles/how-can-i-use-aws-sdk-environment-variables-on-vercel we will need to configure it via env vars. while we still want to use aws cred profiles in other deployements (e.g. bk), this diff does the special treatment for vercel.

Test Plan: preview works: https://dagster-fia92dzji-elementl.vercel.app/getting-started

Reviewers: jordansanders, sashank

Reviewed By: jordansanders

Differential Revision: https://dagster.phacility.com/D8599