CI: Simplify System Tests

[felixge]

What does this PR do?

1. Reuse install_runner action from the system tests repo. This reduces code duplication.
2. Use explicit build steps for runner, weblog and agent. This helps debugging performance.
3. Update to checkout v3. This is just house keeping.

Motivation

Reduce duplication and make it easier to debug performance issues.

Describe how to test/QA your changes

Reviewer's Checklist

• Changed code has unit tests for its functionality.
• If this interacts with the agent in a new way, a system test has been added.

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-06-27 07:17:14

Comparing candidate commit aa698f5 in PR branch felix.geisendoerfer/ci-simplify-system-test with baseline commit 1170127 in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 24 metrics, 0 unstable metrics.