[surrealdb] Install Rust 1.84 to fix build with Option::is_none_or

[emmanuel-keller]

SurrealDB and its dependency tree require stable Rust ≥ 1.84 (Option::is_none_or was stabilised in 1.84).

Builds on the current base-builder image fails with:

error[E0658]: use of unstable library feature 'is_none_or'

This change installs the 1.84.0 toolchain (minimal profile + rust-src) and sets it as the default.

https://issues.oss-fuzz.com/issues/398679984

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[github-actions]

emmanuel-keller is either the primary contact or is in the CCs list of projects/surrealdb.
emmanuel-keller is a new contributor to projects/surrealdb. The PR must be approved by known contributors before it can be merged. The past contributors are: rowan-baker, gguillemas, catenacyber, nathaniel-brough