Increase the benchmark rustc version to 2025-05-28

We may soon want to use some new nightly features in `compiler-builtins`
and `libm`, specifically `cfg_target_has_reliable_f16_f128` which was
added in the past few weeks. This will mean we need a newer toolchain
for benchmarks to continue building.

Bump to the current latest nightly so we are not blocked on this down
the line.

Author: tgross35

.github/workflows/main.yaml

@@ -13,7 +13,7 @@ env:
   RUSTDOCFLAGS: -Dwarnings
   RUSTFLAGS: -Dwarnings
   RUST_BACKTRACE: full
-  BENCHMARK_RUSTC: nightly-2025-01-16 # Pin the toolchain for reproducable results
+  BENCHMARK_RUSTC: nightly-2025-05-28 # Pin the toolchain for reproducable results
 
 jobs:
   # Determine which tests should be run based on changed files.