Rollup merge of #138013 - Kobzol:ci-post-merge-analysis, r=marcoieni

Add post-merge analysis CI workflow

This PR adds a post-merge analysis workflow, which was discussed [here](https://rust-lang.zulipchat.com/#narrow/channel/242791-t-infra/topic/Reporting.20test.20suite.20statistics.20after.20merge).

The workflow currently analyzes test suite results from bootstrap metrics. It downloads metrics for all known jobs in the parent and current (HEAD) commit, compares them and prints a truncated diff. It then posts this diff to the merged PR as a comment. Later I also want to add other statistics to the analysis, e.g. changes in CI/bootstrap step durations.

It can be tested locally e.g. using this:
```
cargo run --release --manifest-path src/ci/citool/Cargo.toml post-merge-report 3cb02729ab3c6583a3b1d1845c5e22b674f04b8d fd17deacce374a4185c882795be162e17b557050
```
This uses a slightly older commit as a parent, to have more results in the diff (normally the diff won't be so large).

CC `@jieyouxu`

r? `@marcoieni`

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