build: drop obsolete rust-toolchain.toml reference in constants.env

Author: Copilot

constants.env

@@ -7,7 +7,7 @@
 
 # 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
+# used for static analysis & mutation testing
 RUST_LATEST=1.96.1
 # used for coverage and extended analysis; update on a regular basis
 RUST_NIGHTLY=nightly-2026-05-30