[DO NOT MERGE] Force PR builds to 3.15 / 3.15t for testing

Temporarily overrides limit_pr_builds so PR CI builds 3.15 and 3.15t on
Linux, Linux aarch64 and Windows, exercising the new smoke-test dependency
resolution added in the previous commit.

macOS is left alone, and workflows that request python-versions explicitly
(e.g. test_build_wheels_linux_python_versions.yml) are not overridden.

REVERT THIS COMMIT BEFORE MERGING.

Author: atalman

tools/scripts/generate_binary_build_matrix.py

@@ -474,7 +474,17 @@ def generate_wheels_matrix(
             arches += [XPU]
 
     if limit_pr_builds:
-        python_versions = [python_versions[0]]
+        # TEMPORARY (DO NOT MERGE): force PR builds onto 3.15 / 3.15t so CI
+        # exercises the new smoke-test dependency resolution on Windows and
+        # Linux. Revert before merging.
+        if not explicitly_requested_versions and os in (
+            LINUX,
+            LINUX_AARCH64,
+            WINDOWS,
+        ):
+            python_versions = ["3.15", "3.15t"]
+        else:
+            python_versions = [python_versions[0]]
 
     global WHEEL_CONTAINER_IMAGES