test: get latest version of CIV in aws.sh

instead of manually updating CIV version every once in a while. Get
always the latest version.

In CIV CI, this test runs before any change can be introduced into the
container image, so no unexpedted errors should come from the CIV side.

test/cases/aws.sh

@@ -229,7 +229,7 @@ tee "${TEMPDIR}/resource-file.json" <<EOF
         {
             "ami": "$AMI_IMAGE_ID",
             "region": "us-east-1",
-            "instance_type": "t3a.micro",
+            "instance_type": "t3.medium",
             "username": "$SSH_USER",
             "name": "civ-testing-image"
         }