Install only i18n-report in po-diff job

[mgeisler]

Before we would run the full mdbook-install action, but we actually only need the i18n-report binary.

This PR steals the relevant declaration from #3215 to speed up this part of our CI.

[mgeisler]

Run bazel run @binaries//:i18n-report__i18n-report -- diff po/ new-po/ > $GITHUB_STEP_SUMMARY
Extracting Bazel installation...
Starting local Bazel server (9.1.1) and connecting to it...
INFO: Invocation ID: 8bfc65ea-73bb-48c4-b158-62571aee5beb
Computing main repo mapping:
Computing main repo mapping:
Computing main repo mapping:
Loading:
Loading: 0 packages loaded
ERROR: No repository visible as '@binaries' from main repository
INFO: Elapsed time: 4.654s
INFO: 0 processes.
ERROR: Build did NOT complete successfully
ERROR: Build failed. Not running target

I don't understand why it fails like that... I can run this just fine locally:

% bazel run @binaries//:i18n-report__i18n-report -- --help
INFO: Invocation ID: d34b009c-ba8c-46ee-ade4-e46bc3ec7d12
bazel: Entering directory `/home/mg/.cache/bazel/_bazel_mg/5300e7eab28c9f30004e4618f1a962d5/execroot/_main/'
INFO: Analyzed target @@rules_rust++crate+binaries//:i18n-report__i18n-report (0 packages loaded, 0 targets configured).
INFO: Found 1 target...
bazel: Leaving directory `/home/mg/.cache/bazel/_bazel_mg/5300e7eab28c9f30004e4618f1a962d5/execroot/_main/'
Target @@rules_rust++crate+binaries__i18n-report-0.2.0//:i18n-report__bin up-to-date:
  bazel-bin/external/rules_rust++crate+binaries__i18n-report-0.2.0/i18n-report__bin
INFO: Elapsed time: 0.097s, Critical Path: 0.00s
INFO: 1 process: 17 action cache hit, 1 internal.
INFO: Build completed successfully, 1 total action
INFO: Running command line: bazel-bin/external/rules_rust++crate+binaries__i18n-report-0.2.0/i18n-report__bin <args omitted>
Usage: i18n-report__bin <COMMAND>

Commands:
  report  Generate an HTML report about the status of translations in each of the given language files
  diff    Generate a report showing any difference between two directories of language files
  help    Print this message or the help of the given subcommand(s)

Options:
  -h, --help  Print **help**