tests/envdir-url: Add AWS_DEFAULT_REGION envvar

AWS CLI v2 runs into errors if the region is not defined¹

The AWS_DEFAULT_REGION variable is available in the GH Action CI after
the aws-actions/configure-aws-credentials@v4 step.

¹ <actions/runner-images#2791 (comment)>

tests/envdir-url.t

@@ -24,7 +24,7 @@ Files are populated from NEXTSTRAIN_ENVD_URL.
 
   $ aws s3 cp --quiet env.d.zip "$NEXTSTRAIN_ENVD_URL"
 
-  $ docker run --rm -e NEXTSTRAIN_ENVD_URL --env=AWS_{ACCESS_KEY_ID,SECRET_ACCESS_KEY,SESSION_TOKEN} "$IMAGE" \
+  $ docker run --rm -e NEXTSTRAIN_ENVD_URL --env=AWS_{ACCESS_KEY_ID,SECRET_ACCESS_KEY,SESSION_TOKEN,DEFAULT_REGION} "$IMAGE" \
   >   bash -eu -c 'echo "$a"; echo "$b"'
   download: s3://nextstrain-tmp/*.zip to ../env.d.zip (glob)
   Archive:  /nextstrain/env.d.zip
@@ -37,7 +37,7 @@ Files are populated from NEXTSTRAIN_ENVD_URL.
 
 Files and NEXTSTRAIN_ENVD_URL are removed with NEXTSTRAIN_DELETE_ENVD=1.
 
-  $ docker run --rm -e NEXTSTRAIN_DELETE_ENVD=1 -e NEXTSTRAIN_ENVD_URL --env=AWS_{ACCESS_KEY_ID,SECRET_ACCESS_KEY,SESSION_TOKEN} "$IMAGE" \
+  $ docker run --rm -e NEXTSTRAIN_DELETE_ENVD=1 -e NEXTSTRAIN_ENVD_URL --env=AWS_{ACCESS_KEY_ID,SECRET_ACCESS_KEY,SESSION_TOKEN,DEFAULT_REGION} "$IMAGE" \
   >   bash -eu -c 'echo "$a"; echo "$b"; ls -1 /nextstrain/env.d'
   download: s3://nextstrain-tmp/*.zip to ../env.d.zip (glob)
   Archive:  /nextstrain/env.d.zip