Fix a typo in the credentials logic (#1257)
Fix a typo in the credentials logic (#1257)

Causes an exception when accessing S3 if the user has never logged into the registry.
quiltdata/quilt-compiler,quiltdata/quilt-compiler,quiltdata/quilt-compiler,quiltdata/quilt,quiltdata/quilt-compiler,quiltdata/quilt,quiltdata/quilt,quiltdata/quilt,quiltdata/quilt