ci: disable CodSpeed process isolation for Rust simulation benchmarks (#10796)

## Summary

`codspeed` 6.0.0 (#10784, #10785) adds process isolation, which is on by
default. `codspeed-divan-compat` now runs each benchmark in a new copy
of the benchmark binary. Simulation mode measures one iteration without
warmup, so each benchmark also pays for allocator setup (mimalloc) and
for simulated cache misses on memory it touches for the first time.
Before, an earlier benchmark in the same process had already paid these
costs.

The CodSpeed report on #10785 shows a near-constant ~15 µs added to
short benchmarks.

Author: robert3005

.github/workflows/codspeed.yml

@@ -116,6 +116,12 @@ jobs:
         run: cargo codspeed build --locked ${{ matrix.features }} $(printf -- '-p %s ' ${{ matrix.packages }}) ${{ steps.select.outputs.benches }} --profile bench
       - name: Run benchmarks
         uses: CodSpeedHQ/action@78102a468f96d8e0ccb92d0ec49425753fc42062  # v5
+        env:
+          # codspeed-divan-compat 6 runs each benchmark in a new process by default. Simulation
+          # measures one iteration without warmup, so each benchmark then also pays for a cold
+          # allocator and cold caches. This adds about 15 µs to each benchmark and hides small
+          # changes in short benchmarks. Run all benchmarks of a target in one process, as before.
+          CODSPEED_PROCESS_ISOLATION: "false"
         with:
           run: cargo codspeed run
           token: ${{ secrets.CODSPEED_TOKEN }}