ggshield: add riscv64 build for 1.56.1 - #4754

_publish-wheel.yml now requires a matrix input (main's "publish:
Validate effective wheel targets"), but this PR branch forked before
that landed and ggshield has never been merged to main, so
build-ggshield.yml never picked up the matching one-line update every
already-published package's workflow got. The merge ref combines the
new required-input _publish-wheel.yml with this branch's old
build-ggshield.yml, so any run has failed startup with "Input matrix is
required, but not provided".

Pass it the same way the mass-update commit did for every other
package's publish job.

rustup-init was run with --default-toolchain none, relying on
rust/rust-toolchain.toml's directory-scoped pin (1.96.0) to supply a
toolchain when the dispatcher itself is built. That pin only applies
inside the ggshield checkout, though, and smoke-test-wheel installs
the built wheel into a fresh venv elsewhere: when pydantic-core or
rfc3161-client resolve to a release newer than pypi.riseproject.dev
has a riscv64 wheel for, pip falls back to building it from its sdist
there, and cargo has no default toolchain to fall back to -
"rustup could not choose a version of cargo to run, because one
wasn't specified explicitly, and no default is configured."

Set --default-toolchain stable instead, so any Rust sdist pip needs to
build outside the ggshield tree has a usable default; the dispatcher
build itself is unaffected, since rust-toolchain.toml's override still
takes precedence inside that tree.