Update lambda/sigterm_test to use RIE via public.ecr.aws/lambda/provided rather than install in the CI environment

[bmoffatt]

Description of changes:

Took this approach in #600 TestConcurrencyWithRIE because the runtime-interface-eumulator hard-codes several paths and gid/uid in managed instances mode. This made using the emulator outside of the Lambda images difficult.

So updating the sigterm test, follow the approach for consistency, and to simplify the CI/CD configuration.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.59%. Comparing base (de51f68) to head (948c6b9).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #607      +/-   ##
==========================================
+ Coverage   73.30%   73.59%   +0.29%     
==========================================
  Files          26       35       +9     
  Lines        1487     1337     -150     
==========================================
- Hits         1090      984     -106     
+ Misses        324      277      -47     
- Partials       73       76       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.