test(sources/s3): fix missing region error

[Juneezee]

Description:

Fixes #4069.

@rosecodym Apologies. I overlooked this and mistakenly assumed that the CI environment had the AWS_REGION variable set. Unfortunately, it doesn't, and that caused the failure on the main branch. This PR should fix that.

https://github.com/trufflesecurity/trufflehog/actions/runs/14910724831/job/41884134392#step:6:1277

=== FAIL: pkg/sources/s3 TestSource_Chunks/gets_chunks_after_assuming_role (unknown)
2025-05-08T16:06:01Z	info-0	context	Creating checkpointer	{"timeout": 30}
    s3_test.go:123: Source.Chunks() error = role "arn:aws:iam::619888638459:role/s3-test-assume-role" could not list any s3 buckets for scanning: operation error S3: ListBuckets, get identity: get credentials: failed to refresh cached credentials, operation error STS: AssumeRole, failed to resolve service endpoint, endpoint rule error, Invalid Configuration: Missing Region, wantErr false

Checklist:

• Tests passing (make test-community)?
• Lint passing (make lint this requires golangci-lint)?

[ahrav]

Thank you @Juneezee