feat(lambda): give function containers the AWS environment

[Sorttech]

Real Lambda injects region and execution-role credentials, and function
code relies on them: an SDK client built with no region or credentials
fails before it reaches an endpoint. fakecloud injected none of the
standard variables, only the function's own environment, so handler code
that called AWS did nothing useful -- CDK's BucketDeployment reported
success having copied no files.

Inject AWS_REGION, AWS_DEFAULT_REGION and credentials, plus
AWS_ENDPOINT_URL pointing at the backend's host alias and this server's
port. The endpoint is the one deliberate deviation from AWS: there the
variable is absent and the SDK's defaults are correct, whereas here the
container has to be pointed back at fakecloud or the handler reaches out
to the real internet. localhost would resolve to the container itself,
so the existing host alias is reused. Defaults are emitted before the
function's own environment, so a function that sets any of them wins.

The E2E asserts the environment the container actually observes. It stops
short of a full SDK round-trip from inside the container: that also needs
container-to-host reachability on an arbitrary host port, which does not
hold on every developer machine (WSL2 here), and would make the test flaky
for reasons unrelated to this change.

Test plan

Regression test: lambda_container_receives_the_aws_environment in crates/fakecloud-e2e/tests/lambda_aws_env.rs

Verification

On this exact head, rebased onto current main:

• cargo fmt --all --check - clean.
• cargo clippy --workspace --all-targets -- -D warnings - clean.
• cargo test --workspace excluding fakecloud-e2e, fakecloud-conformance,
  fakecloud-tfacc and fakecloud-parity (the same set CI's test job runs),
  plus cargo test -p fakecloud-conformance --lib - 9931 passed, 0 failed.
• cargo test -p fakecloud-e2e --test lambda_aws_env - the regression test
  above passes in a real Lambda container under Docker against a freshly
  built target/debug/fakecloud.

Found by deploying a CDK CloudFront + S3 SPA against fakecloud.

[Sorttech]

Retargeted upstream as faiscadev#2562.