ci: bump pinned toolchain 1.89.0 -> 1.91.0

CI is red on every job, but not from this change: the benchmark/diff jobs
fail installing bencher because its current source uses Duration::from_hours,
which is only const-stable on Rust >= 1.91, while rust-toolchain.toml pins
1.89.0. The build/test matrix also trips over the pinned toolchain (clippy
component missing) and gets cancelled by fail-fast. Bump the pin to 1.91.0
and declare the clippy/rustfmt components so the toolchain is complete.

Author: ianm199

rust-toolchain.toml

@@ -1,2 +1,3 @@
 [toolchain]
-channel = "1.89.0"
+channel = "1.91.0"
+components = ["clippy", "rustfmt"]