AZP: target ucxx main commit instead of release tag

Pin the rapidsai/ucxx resource to a specific main commit (33deb0b) rather
than v0.51.00a. Alpha tags are cut at code-freeze and don't pick up ongoing
main work, and an old tag drifts from RAPIDS CI updates (images, ci/ scripts)
that must move in tandem. A pinned commit stays immutable/reproducible while
letting us do controlled bumps. This commit already includes ucxx #674, so
drop the local <unistd.h> patch in build_ucxx.sh + test_ucxx.sh.

Author: Alexey-Rivkin

buildlib/azure-pipelines-pr.yml

@@ -37,7 +37,7 @@ resources:
       type: github
       name: rapidsai/ucxx
       endpoint: Mellanox-lab
-      ref: refs/tags/v0.51.00a
+      ref: 33deb0b581b78027730e8ef86ed32efbb22d0dd8
 
 extends:
   template: pr/main.yml

buildlib/tools/build_ucxx.sh

@@ -79,13 +79,6 @@ if [[ "$phase" == wheel_* ]]; then
   export CC="$toolset/gcc" CXX="$toolset/g++"
 fi
 
-# Upstream ucxx header uses usleep() but omits <unistd.h>; undeclared on
-# newer gcc. Affects all C++ phases.
-if [[ "$phase" != docs ]]; then
-  hdr=python/ucxx/ucxx/examples/python_future_task.h
-  grep -q "include <unistd.h>" "$hdr" || sed -i '/^#pragma once/a #include <unistd.h>' "$hdr"
-fi
-
 case "$phase" in
   conda_cpp)              bash ci/build_cpp.sh ;;
   conda_python)           bash ci/build_python.sh ;;

buildlib/tools/test_ucxx.sh

@@ -49,10 +49,6 @@ done
 
 case "$phase" in
   build)
-    # Upstream ucxx examples header uses usleep() but omits <unistd.h>;
-    # undeclared on newer gcc. Same patch as build_ucxx.sh.
-    hdr=python/ucxx/ucxx/examples/python_future_task.h
-    grep -q "include <unistd.h>" "$hdr" || sed -i '/^#pragma once/a #include <unistd.h>' "$hdr"
     if [ "${IS_GPU,,}" = "true" ]; then
       # sccache wrapper crashes CMake's compiler probe on the GPU build hosts; no-op it.
       cat > "$HOME/.local/bin/rapids-configure-sccache" <<'EOF'