wheel builds: react to changes in pip's handling of build constraints (rapidsai#1710)

Contributes to rapidsai/build-planning#242

Modifying `ci/build_wheel.sh` to

- pass`--build-constraint="${PIP_CONSTRAINT}"` unless build isolation is enabled.
- unset `PIP_CONSTRAINT` (set by rapids-init-pip)... it doesn't affect builds as of pip 25.3, and results in an error from `pip wheel` when set and `--build-constraint` is also passed

Authors:
  - Mike McCarty (https://github.com/mmccarty)

Approvers:
  - James Lamb (https://github.com/jameslamb)

URL: rapidsai#1710

Author: mmccarty

ci/build_wheel.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
-# SPDX-FileCopyrightText: Copyright (c) 2023-2025, NVIDIA CORPORATION.
+# SPDX-FileCopyrightText: Copyright (c) 2023-2026, NVIDIA CORPORATION.
 # SPDX-License-Identifier: Apache-2.0
 
 set -euo pipefail
@@ -47,11 +47,26 @@ rapids-logger "Building '${package_name}' wheel"
 
 sccache --stop-server 2>/dev/null || true
 
+RAPIDS_PIP_WHEEL_ARGS=(
+  -w dist
+  -v
+  --no-deps
+  --disable-pip-version-check
+)
+# Only use --build-constraint when build isolation is enabled.
+#
+# Passing '--build-constraint' and '--no-build-isolation` together results in an error from 'pip',
+# but we want to keep environment variable PIP_CONSTRAINT set unconditionally.
+# PIP_NO_BUILD_ISOLATION=0 means "add --no-build-isolation" (ref: https://github.com/pypa/pip/issues/573
+if [[ "${PIP_NO_BUILD_ISOLATION:-}" != "0" ]]; then
+    RAPIDS_PIP_WHEEL_ARGS+=(--build-constraint="${PIP_CONSTRAINT}")
+fi
+
+# unset PIP_CONSTRAINT (set by rapids-init-pip)... it doesn't affect builds as of pip 25.3, and
+# results in an error from 'pip wheel' when set and --build-constraint is also passed
+unset PIP_CONSTRAINT
 rapids-pip-retry wheel \
-    -w dist \
-    -v \
-    --no-deps \
-    --disable-pip-version-check \
+    "${RAPIDS_PIP_WHEEL_ARGS[@]}" \
     .
 
 sccache --show-adv-stats

ci/build_wheel_cuvs.sh

@@ -1,5 +1,5 @@
 #!/bin/bash
-# SPDX-FileCopyrightText: Copyright (c) 2023-2025, NVIDIA CORPORATION.
+# SPDX-FileCopyrightText: Copyright (c) 2023-2026, NVIDIA CORPORATION.
 # SPDX-License-Identifier: Apache-2.0
 
 set -euo pipefail
@@ -13,8 +13,8 @@ RAPIDS_PY_CUDA_SUFFIX="$(rapids-wheel-ctk-name-gen "${RAPIDS_CUDA_VERSION}")"
 # Downloads libcuvs wheels from this current build,
 # then ensures 'cuvs' wheel builds always use the 'libcuvs' just built in the same CI run.
 #
-# Using env variable PIP_CONSTRAINT is necessary to ensure the constraints
-# Using env variable PIP_CONSTRAINT (initialized by 'rapids-init-pip') is necessary to ensure the constraints
+# env variable 'PIP_CONSTRAINT' is set up by rapids-init-pip. It constrains all subsequent
+# 'pip install', 'pip download', etc. calls (except those used in 'pip wheel', handled separately in build scripts)
 LIBCUVS_WHEELHOUSE=$(RAPIDS_PY_WHEEL_NAME="libcuvs_${RAPIDS_PY_CUDA_SUFFIX}" rapids-download-wheels-from-github cpp)
 echo "libcuvs-${RAPIDS_PY_CUDA_SUFFIX} @ file://$(echo "${LIBCUVS_WHEELHOUSE}"/libcuvs_*.whl)" >> "${PIP_CONSTRAINT}"