build: upgrade Rust toolchain to 1.96.1

Pins RUST_LATEST to 1.96.1 to pull in Cargo bugfixes, notably proper HTTP retries.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: sandersaares

constants.env

@@ -8,7 +8,7 @@
 # used for testing, ensures the MSRV promise is kept; must match Cargo.toml [workspace.package].rust-version
 RUST_MSRV=1.93
 # used for static analysis & mutation testing; must match rust-toolchain.toml
-RUST_LATEST=1.96
+RUST_LATEST=1.96.1
 # used for coverage and extended analysis; update on a regular basis
 RUST_NIGHTLY=nightly-2026-05-30
 # used for external type exposure checks; update alongside updates to cargo-check-external-types