Rollup merge of #138531 - Kobzol:test-diff-try-build, r=marcoieni

Store test diffs in job summaries and improve analysis formatting

This PR stores the test diffs that we already have in the post-merge workflow also into individual job summaries. This makes it easier to compare test (and later also other) diffs per job, which will be especially useful for try jobs, so that we can actually see the test diffs *before* we merge a given PR.

As a drive-by, I also made a bunch of cleanups in `citool` and in the formatting of the summary and post-merge analyses. These changes are split into self-contained commits.

The analysis can be tested locally with the following command:
```bash
$ curl https://ci-artifacts.rust-lang.org/rustc-builds/<current-sha>/metrics-<job-name>.json > metrics.json
$ cargo run --manifest-path src/ci/citool/Cargo.toml postprocess-metrics metrics.json --job-name <job-name> --parent <parent-sha> > out.md
```
For example, for [this PR](rust-lang/rust#138523):
```bash
$ curl https://ci-artifacts.rust-lang.org/rustc-builds/282865097d138c7f0f7a7566db5b761312dd145c/metrics-aarch64-gnu.json > metrics.json
$ cargo run --manifest-path src/ci/citool/Cargo.toml postprocess-metrics metrics.json --job-name aarch64-gnu --parent d9e5539a39192028a7b15ae596a8685017faecee > out.md
```

Best reviewed commit by commit.

r? `@marcoieni`

try-job: aarch64-gnu
try-job: dist-x86_64-linux

Author: matthiaskrgr