Windows: skip site-packages copy when source dir is missing + propagate

flutter test failure on Windows runner.

Two bugs surfaced in the previous Windows CI run:

1. The workflow-level env var SERIOUS_PYTHON_SITE_PACKAGES is set, but the
   directory it points at only exists when an outer step (the old
   cibuildwheel-based workflow) populated it. With the new pre-built-binary
   pipeline that step is gone, so cmake -E copy_directory ran against a
   missing source and CopyPythonDLLs.vcxproj exited 1. Guard with
   `if(DEFINED ENV{...} AND EXISTS "$ENV{...}")`.

2. The Windows job's GitHub Actions step used the default PowerShell shell,
   where `flutter test ... | tail -200` swallows flutter's non-zero exit
   (PowerShell pipelines don't honor pipefail). Tests reported "0 tests
   passed, 1 failed" yet the step reported success. Switch to `shell: bash`
   + explicit `set -o pipefail` so failures surface.

Author: FeodorFitsner

.github/workflows/test-bridge-build.yml

@@ -197,9 +197,14 @@ jobs:
 
       - name: Package + run integration test
         working-directory: "src/serious_python/example/bridge_example"
+        shell: bash
         run: |
           dart run serious_python:main package app/src --platform Windows --python-version ${{ matrix.python_version }}
-          flutter test integration_test -d windows -v 2>&1 | tail -200
+          # bash + pipefail so the test step actually fails when flutter fails
+          # (default PowerShell shell on windows runners eats the failure when
+          # piping into tail).
+          set -o pipefail
+          flutter test integration_test -d windows -v 2>&1 | tail -300
 
       - name: Diagnostics on failure
         if: failure()

src/serious_python_windows/windows/CMakeLists.txt

@@ -139,14 +139,17 @@ add_custom_command(TARGET CopyPythonDLLs POST_BUILD
     -P "${CMAKE_CURRENT_SOURCE_DIR}/rm_globs.cmake"
 )
 
-if(DEFINED ENV{SERIOUS_PYTHON_SITE_PACKAGES})
+# Mirror the user's external site-packages dir into runner/site-packages if
+# the env var is set AND points at an existing directory. (The CI env var
+# may be set workflow-wide while specific jobs don't actually create the
+# directory — `cmake -E copy_directory` against a missing source fails,
+# which used to wedge CopyPythonDLLs.)
+if(DEFINED ENV{SERIOUS_PYTHON_SITE_PACKAGES} AND EXISTS "$ENV{SERIOUS_PYTHON_SITE_PACKAGES}")
   add_custom_command(TARGET CopyPythonDLLs POST_BUILD
-    COMMAND ${CMAKE_COMMAND} -E remove_directory
-      "${CMAKE_BINARY_DIR}/runner/$<$<CONFIG:Release>:Release>$<$<CONFIG:Debug>:Debug>/site-packages"
-    COMMAND ${CMAKE_COMMAND} -E make_directory
-      "${CMAKE_BINARY_DIR}/runner/$<$<CONFIG:Release>:Release>$<$<CONFIG:Debug>:Debug>/site-packages"
+    COMMAND ${CMAKE_COMMAND} -E rm -rf "${SP_RUNNER_OUT}/site-packages"
+    COMMAND ${CMAKE_COMMAND} -E make_directory "${SP_RUNNER_OUT}/site-packages"
     COMMAND ${CMAKE_COMMAND} -E copy_directory
       "$ENV{SERIOUS_PYTHON_SITE_PACKAGES}"
-      "${CMAKE_BINARY_DIR}/runner/$<$<CONFIG:Release>:Release>$<$<CONFIG:Debug>:Debug>/site-packages"
+      "${SP_RUNNER_OUT}/site-packages"
   )
 endif()