reclient_helper: tuning local execution parameter

With this CL, build time of chrome with non-cache hit case improved a
bit on P620 Linux workstation.

```
Benchmark 1: RBE_local_resource_fraction=0.4 RBE_racing_bias=0.7 RBE_remote_accept_cache=false autoninja -C out/Default chrome
  Time (mean ± σ):     337.697 s ± 11.439 s    [User: 3086.301 s, System: 1362.952 s]
  Range (min … max):   327.732 s … 351.680 s    5 runs
```

```
Benchmark 1: RBE_remote_accept_cache=false autoninja -C out/Default chrome
  Time (mean ± σ):     347.795 s ± 15.906 s    [User: 2764.181 s, System: 1199.244 s]
  Range (min … max):   331.964 s … 367.898 s    5 runs
```

Bug: 352463976
Change-Id: Ic9732583857ce175888a7ec65cf9e323529f0a07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5693250
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Junji Watanabe <jwata@google.com>
Auto-Submit: Takuto Ikuta <tikuta@chromium.org>

reclient_helper.py

@@ -260,8 +260,13 @@ def set_reproxy_path_flags(out_dir, make_dirs=True):
 
 
 def set_racing_defaults():
-    os.environ.setdefault("RBE_local_resource_fraction", "0.2")
-    os.environ.setdefault("RBE_racing_bias", "0.95")
+    if sys.platform == 'linux':
+        os.environ.setdefault("RBE_local_resource_fraction", "0.4")
+        os.environ.setdefault("RBE_racing_bias", "0.7")
+    else:
+        # TODO(b/352463976): tune this on non-Linux platform too.
+        os.environ.setdefault("RBE_local_resource_fraction", "0.2")
+        os.environ.setdefault("RBE_racing_bias", "0.95")
 
 
 def set_mac_defaults():