Build the Tracy-enable iree-runtime package by default. (#14732)

This has been stable for the last week on CI/interactive use. Flipping
to default as it is always supported.

`IREE_RUNTIME_BUILD_TRACY_TOOLS=ON` remains opt-in because that requires
system dependencies to be met (this gates the availability of
iree-tracy-capture). It is manually enabled in CI on platforms that
support it.
diff --git a/runtime/setup.py b/runtime/setup.py
index 4b8cbc4..cd974d0 100644
--- a/runtime/setup.py
+++ b/runtime/setup.py
@@ -41,8 +41,8 @@
 from setuptools.command.build_py import build_py as _build_py
 
 
-def getenv_bool(key):
-    value = os.getenv(key, "OFF")
+def getenv_bool(key, default_value="OFF"):
+    value = os.getenv(key, default_value)
     return value.upper() in ["ON", "1", "TRUE"]
 
 
@@ -53,9 +53,12 @@
     return result
 
 
-ENABLE_TRACY = getenv_bool("IREE_RUNTIME_BUILD_TRACY")
+ENABLE_TRACY = getenv_bool("IREE_RUNTIME_BUILD_TRACY", "ON")
 if ENABLE_TRACY:
-    print("*** Enabling Tracy instrumented runtime", file=sys.stderr)
+    print(
+        "*** Enabling Tracy instrumented runtime (disable with IREE_RUNTIME_BUILD_TRACY=OFF)",
+        file=sys.stderr,
+    )
 else:
     print(
         "*** Tracy instrumented runtime not enabled (enable with IREE_RUNTIME_BUILD_TRACY=ON)",