Gate runtime-async behind 'runtimeasync' experiment

PR #5195 enabled the runtime-async language feature unconditionally for
net11.0+ via `<Features>`. To avoid affecting baseline measurement runs,
gate it behind the existing experiment infrastructure so it only takes
effect in the dedicated experiment lane.

- src/Directory.Build.targets: only set `runtime-async=on` when the
  `EnableRuntimeAsync` MSBuild property is `true` (in addition to the
  existing TFM check).
- scripts/ci_setup.py: when `--experiment-name=runtimeasync` is passed,
  emit `EnableRuntimeAsync=true` as an env var so MSBuild picks it up
  as a property (matches the pattern used by the `jitoptrepeat` experiment).

Verified locally: BDN dry runs against the BinaryDataPayload tests
succeed both with and without the env var (18/18 benchmarks each).

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

Author: DrewScoggins

scripts/ci_setup.py

@@ -424,6 +424,11 @@ def main(args: CiSetupArgs):
     if args.experiment_name == "jitoptrepeat":
         experiment_config = variable_format % ('DOTNET_JitOptRepeat', '*')
 
+    if args.experiment_name == "runtimeasync":
+        # Surfaced to MSBuild as the $(EnableRuntimeAsync) property; gates the
+        # runtime-async Features flag in src/Directory.Build.targets.
+        experiment_config = variable_format % ('EnableRuntimeAsync', 'true')
+
     output = ''
 
     with push_dir(get_repo_root_path()):

src/Directory.Build.targets

@@ -2,6 +2,10 @@
     <Import Project="../Directory.Build.targets" />
 
     <PropertyGroup>
-      <Features Condition="$([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net11.0'))">$(Features);runtime-async=on</Features>
+      <!-- Runtime-async is gated behind the 'runtimeasync' experiment so it only runs
+           in the dedicated experiment lane. ci_setup.py sets EnableRuntimeAsync=true as
+           an env var (which MSBuild surfaces as a property) when the experiment name
+           is 'runtimeasync'. -->
+      <Features Condition="'$(EnableRuntimeAsync)' == 'true' and $([MSBuild]::IsTargetFrameworkCompatible('$(TargetFramework)', 'net11.0'))">$(Features);runtime-async=on</Features>
     </PropertyGroup>
 </Project>