Bump cibuildwheel to 4.2.0 to fix musllinux riscv64 wheels - #1146

The 0.21.1 to 0.22.0 changelog batch (msgspec#1112) landed on 2026-07-03. Four
user-visible changes merged between then and the 0.22.0 release on
2026-08-11 never got an entry:

- msgspec#700, overloads on the `Meta` stub, so type checkers reject mixing
`gt` with `ge` and `lt` with `le`
- msgspec#1028, `null` placed last in the `anyOf` generated for optional unions
- msgspec#1114, `__struct_encode_fields__` added to `src/msgspec/__init__.pyi`
(its companion msgspec#813 only touched `tests/typing`, so it needs no entry of
its own)
- msgspec#1118, `json.encode` raising `TypeError` for a `dict` keyed by a plain
`str`-valued `enum.Enum`

I went over every PR merged in that window. The rest need no entry: msgspec#813
and msgspec#1117 are tests only, msgspec#1121 fixes an unreleased regression from
msgspec#1028, msgspec#1145 is an unused variable under free-threaded builds, and msgspec#1146
and msgspec#1148 are CI and tooling. msgspec#1127, msgspec#1135 and msgspec#1080 already have
entries.

Docs build clean with `--fail-on-warning`.

Note for whoever retries the release: the section is still headed
`Version 0.22.0 (2026-08-11)`, and that date will need updating, since
the tag was deleted after the upload failed. Refs msgspec#1134.

Co-authored-by: Siyet <[email protected]>

`build-wheels.yml` skips every musllinux wheel unless `build-all` is
true, and `ci.yml` only passes `build-all` on release. So no musllinux
wheel, on any architecture, is built outside a release, and the first
run that can show a musllinux breakage is the one meant to publish.
msgspec#1134 hit exactly that when `cp310-musllinux_riscv64` failed on the
0.22.0 release build, and msgspec#1146 could not be exercised by its own CI for
the same reason.

**Change.** `ci.yml` gains a weekly `schedule` trigger (Monday 04:17
UTC) and passes `build-all` on `schedule` as well as on `release`, so
the complete matrix, musllinux and riscv64 included, runs on a timer
between releases. Pull request and push runs are unchanged, and so is
the `build-riscv64` label gate. Five lines, no new jobs.

**What proves it.** A workflow cannot run locally; the file parses as
YAML. The first scheduled run showing the musllinux and riscv64 legs
outside a release is the proof. If PR-time coverage is wanted as well, a
single `cp312-musllinux_x86_64` smoke build gated on changes to the
wheel workflow, `setup.py`, `pyproject.toml` or `src/` is a small
follow-up; I left it out of this change so the schedule can land on its
own.

One question for the reviewer: is weekly the right cadence for the
project's CI budget, or would you rather tie the full build to a
pre-release tag?

Closes msgspec#1150

msgspec#1146 bumped cibuildwheel to
4.2.0, which adds CPython 3.15 wheels by default, but this project uses
a build list in `.cibuildwheel.toml` so they need to be selected
explicitly.

Signed-off-by: Edgar Ramírez Mondragón <[email protected]>