More CI improvements (#8313)

* Reduce clutter in log of Python test

* Set up BuildKite test analytics

* Add separate step for building containers

* Enable incremental update of CI stack; custom agent IAM policy

tests/buildkite/infrastructure/aws-stack-creator/agent-iam-policy-template.yml

@@ -0,0 +1,32 @@
+---
+AWSTemplateFormatVersion: "2010-09-09"
+Description: "Buildkite agent's IAM policy"
+
+Resources:
+  BuildkiteAgentManagedPolicy:
+    Type: AWS::IAM::ManagedPolicy
+    Properties:
+      PolicyDocument:
+        {
+          "Version": "2012-10-17",
+          "Statement": [
+            {
+              "Effect": "Allow",
+              "Action": [
+                "s3:*",
+                "s3-object-lambda:*"
+              ],
+              "Resource": "*"
+            },
+            {
+              "Effect": "Allow",
+              "Action": "lambda:InvokeFunction",
+              "Resource": "*"
+            },
+            {
+              "Effect": "Allow",
+              "Action": "secretsmanager:GetSecretValue",
+              "Resource": "*"
+            }
+          ]
+        }