Fix build issues during import.

When stdexec is used as a 3rd-party dependency for example with fetch
content it is downloaded into the cmake build folder under the _deps.

A setup like that will results a build directory structure like
build/_deps/stdexec-src
           /stdexec-build

in this context the CMAKE_BINARY_DIR is the build folder and the
CMAKE_CURRENT_BINARY_DIR is the build/_deps/stdexec-build.

Issue #1:
Right now the build will place two files into the 'main' build folder:
 - execution.bs
 - RAPIDS.cmake

It might causes problems when the main cmake project uses RAPIDS
with a different version.

Issue NVIDIA#2: There is no such a file stdexec_version_config.hpp under the
main build tree. And it will cause a configuration error.

Author: David Eles

CMakeLists.txt

@@ -18,12 +18,12 @@ endif()
 # - Download and initialize RAPIDS CMake helpers -----------------------------
 
 # Fetch rapids-cmake
-if(NOT EXISTS ${CMAKE_BINARY_DIR}/RAPIDS.cmake)
+if(NOT EXISTS ${CMAKE_CURRENT_BINARY_DIR}/RAPIDS.cmake)
   file(DOWNLOAD https://raw.githubusercontent.com/rapidsai/rapids-cmake/branch-24.02/RAPIDS.cmake
-       ${CMAKE_BINARY_DIR}/RAPIDS.cmake)
+       ${CMAKE_CURRENT_BINARY_DIR}/RAPIDS.cmake)
 endif()
 # Initialize rapids-cmake
-include(${CMAKE_BINARY_DIR}/RAPIDS.cmake)
+include(${CMAKE_CURRENT_BINARY_DIR}/RAPIDS.cmake)
 # utilities for generating export set package metadata
 include(rapids-export)
 # utilities for finding packages
@@ -37,11 +37,11 @@ include(rapids-cpm)
 # - Project definition -------------------------------------------------------
 
 # Define the project and set the version and languages
-if(NOT EXISTS ${CMAKE_BINARY_DIR}/execution.bs)
+if(NOT EXISTS ${CMAKE_CURRENT_BINARY_DIR}/execution.bs)
   file(DOWNLOAD "https://raw.githubusercontent.com/cplusplus/sender-receiver/main/execution.bs"
-      ${CMAKE_BINARY_DIR}/execution.bs)
+      ${CMAKE_CURRENT_BINARY_DIR}/execution.bs)
 endif()
-file(STRINGS "${CMAKE_BINARY_DIR}/execution.bs" STD_EXECUTION_BS_REVISION_LINE REGEX "Revision: [0-9]+")
+file(STRINGS "${CMAKE_CURRENT_BINARY_DIR}/execution.bs" STD_EXECUTION_BS_REVISION_LINE REGEX "Revision: [0-9]+")
 string(REGEX REPLACE "Revision: ([0-9]+)" "\\1" STD_EXECUTION_BS_REVISION ${STD_EXECUTION_BS_REVISION_LINE})
 
 # nvc++ isn't supported by (s)ccache yet, so unset these before the `project()`
@@ -164,9 +164,9 @@ PUBLIC
   # stdexec_version_config.hpp is generated by rapids' script
   FILE_SET version_config
   TYPE HEADERS
-  BASE_DIRS ${CMAKE_BINARY_DIR}/include
+  BASE_DIRS ${CMAKE_CURRENT_BINARY_DIR}/include
   FILES
-    ${CMAKE_BINARY_DIR}/include/stdexec_version_config.hpp
+    ${CMAKE_CURRENT_BINARY_DIR}/include/stdexec_version_config.hpp
 )
 list(APPEND stdexec_export_targets stdexec)