use found python executable for launching sphinx-build

there is chance that the distro install python as python3, in that case
the shebang of
```
```
in build.py won't work.
Signed-off-by: Kefu Chai <tchaikov@gmail.com>

doc/CMakeLists.txt

@@ -4,11 +4,13 @@ if (NOT DOXYGEN)
   return ()
 endif ()
 
+find_package(PythonInterp
+  QUIET REQUIRED)
 find_program(SPHINX_EXECUTABLE
   NAMES sphinx-build sphinx-build-3)
 add_custom_target(doc
   COMMAND ${CMAKE_COMMAND} -E env SPHINX_EXECUTABLE=${SPHINX_EXECUTABLE}
-    ${CMAKE_CURRENT_SOURCE_DIR}/build.py ${FMT_VERSION})
+    ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_SOURCE_DIR}/build.py ${FMT_VERSION}
   SOURCES api.rst syntax.rst build.py conf.py _templates/layout.html)
 
 install(DIRECTORY ${CMAKE_CURRENT_BINARY_DIR}/html/