🛠️ Gizmos: Gizmos that don't use S3 shouldn't fail w/o AWS

- #709

We initialize `active-storage` and `aws-sdk-s3` when we load the
application code, which loads up `config/storage.yml` and sets up an AWS
S3 Client.

But the `Journal` doesn't use AWS S3 (yet?), so the continuous
integration environment doesn't have AWS credentials. Which causes
[build failures].

I *could* add credentials to the Journal Gizmo's CI environment, but
that would mean other Gizmos that don't use AWS S3 would need AWS
credentials

So I'm going to add some fake test credentials to the .env.test.example
file, which should let the AWS S3 client initialize and then explode
horribly when we try and shovel some data into it.

[build failures]: https://github.com/zinc-collective/convene-journal/actions/runs/7690936384/job/20955491503

.env.test.example

@@ -2,3 +2,7 @@
 MARKETPLACE_VENDOR_STRIPE_ACCOUNT=acct_GET_THIS_FROM_STRIPE
 MARKETPLACE_VENDOR_SQUARE_ACCESS_TOKEN=GET_THIS_FROM_SQUARE
 MARKETPLACE_VENDOR_SQUARE_LOCATION_ID=GET_THIS_FROM_SQUARE
+
+AWS_S3_ACCESS_KEY_ID="GET_THIS_FROM_AWS"
+AWS_S3_SECRET_ACCESS_KEY="GET_THIS_FROM_AWS"
+AWS_S3_BUCKET="GET_THIS_FROM_AWS"