chore(deps): update rust docker tag to v1.99

[ferrlabs-renovate]

This PR contains the following updates:

Package	Type	Update	Change
rust (source)	stage	minor	1.98-alpine → 1.99-alpine

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[ferrfleet]

Builder image bump from rust 1.98 to 1.99, still pinned by digest. rust-toolchain.toml tracks stable and there's no rust-version pin, so nothing else needs to move with it. Nothing blocking. Build Release Binary and Test were still running when I reviewed, so wait for them to go green before merging.

[ferrfleet-interne]

Renovate re-pushed with a new rust:1.99-alpine digest (0cce0a5e… → cbd696e7…). The new one matches the tag's current digest on Docker Hub, which was republished 2026-10-15 19:45 UTC, so this is an upstream image rebuild and not a version change. Nothing blocking. Test, Build Release Binary and Cargo Security were still running when I reviewed this.

[ferrfleet-interne]

Renovate re-pushed with a new rust:1.99-alpine digest (cbd696e7… → 020ae587…). It matches the tag's current digest on Docker Hub, which was republished 2026-10-15 22:45 UTC, so this is another upstream rebuild and the version is unchanged. Nothing blocking.