build: use python from venv if available

First check if $VIRTUAL_ENV is set. If it is, use the python binary from
this venv.

Fall back to system python otherwise.

This helps run various targets which use python, when system python is
not being used.

CMakeLists.txt

@@ -534,13 +534,31 @@ if(WERROR)
   unset(werror_flag)
 endif()
 
-find_package(Python3 3.10 COMPONENTS Interpreter)
-if(Python3_EXECUTABLE)
-  set(PYTHON_COMMAND ${Python3_EXECUTABLE})
-else()
-  list(APPEND configure_warnings
-    "Minimum required Python not found. Utils and rpcauth tests are disabled."
-  )
+# First check for active virtual environment
+if(DEFINED ENV{VIRTUAL_ENV})
+  if(WIN32)
+    if(EXISTS "$ENV{VIRTUAL_ENV}/Scripts/python.exe")
+      set(Python3_EXECUTABLE "$ENV{VIRTUAL_ENV}/Scripts/python.exe")
+      set(PYTHON_COMMAND ${Python3_EXECUTABLE})
+    endif()
+  else()
+    if(EXISTS "$ENV{VIRTUAL_ENV}/bin/python3")
+      set(Python3_EXECUTABLE "$ENV{VIRTUAL_ENV}/bin/python3")
+      set(PYTHON_COMMAND ${Python3_EXECUTABLE})
+    endif()
+  endif()
+endif()
+
+if(NOT DEFINED PYTHON_COMMAND)
+  # Fall back to system Python if no venv is active or found
+  find_package(Python3 3.10 COMPONENTS Interpreter)
+  if(Python3_EXECUTABLE)
+    set(PYTHON_COMMAND ${Python3_EXECUTABLE})
+  else()
+    list(APPEND configure_warnings
+      "Minimum required Python not found. Utils and rpcauth tests are disabled."
+    )
+  endif()
 endif()
 
 target_compile_definitions(core_interface INTERFACE ${DEPENDS_COMPILE_DEFINITIONS})