libstore: Put all the AWS credentials logic behind interface class AwsCredentialProvider

This makes it so we don't need to rely on global variables and hacky destructors to
clean up another global variable. Just putting it in the correct order in the class
is more than enough.

src/libstore/unix/build/derivation-builder.cc

@@ -958,7 +958,7 @@ std::optional<AwsCredentials> DerivationBuilderImpl::preResolveAwsCredentials()
                     auto s3Url = ParsedS3URL::parse(parsedUrl);
 
                     // Use the preResolveAwsCredentials from aws-creds
-                    auto credentials = nix::preResolveAwsCredentials(s3Url);
+                    auto credentials = getAwsCredentialsProvider()->getCredentials(s3Url);
                     debug("Successfully pre-resolved AWS credentials in parent process");
                     return credentials;
                 }