[DO NOT MERGE] Temporarily force PR builds to test 3.15/3.15t on Linux

Temporary commit so this PR's CI exercises the newly added 3.15 and 3.15t
Linux wheels instead of only the oldest version (3.10). PR builds normally
limit to python_versions[0]; this overrides that to ["3.15", "3.15t"] for
linux and linux-aarch64 only (macOS/Windows keep 3.10 to avoid an empty
matrix).

Revert this commit before merging.

Test Plan:

```
python3 -m tools.tests.test_generate_binary_build_matrix
```

All 7 reference tests still pass (they exercise the non-limited path).

This commit was authored with the assistance of an AI assistant.

Author: atalman

tools/scripts/generate_binary_build_matrix.py

@@ -456,7 +456,13 @@ def generate_wheels_matrix(
             arches += [XPU]
 
     if limit_pr_builds:
-        python_versions = [python_versions[0]]
+        # TEMPORARY (DO NOT MERGE): force PR builds to exercise 3.15 / 3.15t on
+        # Linux so this PR's CI validates the new versions. Revert to
+        # `python_versions = [python_versions[0]]` before merging.
+        if os in (LINUX, LINUX_AARCH64):
+            python_versions = ["3.15", "3.15t"]
+        else:
+            python_versions = [python_versions[0]]
 
     global WHEEL_CONTAINER_IMAGES