Revert "Improve FIREBASE_PYTHON_EXECUTABLE cmake cache var and make its usage ubiquitous"

It seemed to cause problems with the build. There is a better way in the iOS SDK anyways.

This reverts commit c6346fd.

Author: dconeybe

CMakeLists.txt

@@ -86,21 +86,8 @@ if (NOT FIREBASE_ANDROID_STL STREQUAL "")
   set(ANDROID_STL ${FIREBASE_ANDROID_STL})
 endif()
 
-# Find a Python interpreter using the best available mechanism.
-if(${CMAKE_VERSION} VERSION_LESS "3.12")
-  include(FindPythonInterp)
-  set(DEFAULT_FIREBASE_PYTHON_EXECUTABLE "${PYTHON_EXECUTABLE}")
-else()
-  find_package(Python3 COMPONENTS Interpreter)
-  set(DEFAULT_FIREBASE_PYTHON_EXECUTABLE "${Python3_EXECUTABLE}")
-endif()
-
-set(
-  FIREBASE_PYTHON_EXECUTABLE
-  "${DEFAULT_FIREBASE_PYTHON_EXECUTABLE}"
-  CACHE FILEPATH
-  "The Python interpreter to use"
-)
+set(FIREBASE_PYTHON_EXECUTABLE "python" CACHE FILEPATH
+  "The Python interpreter to use, such as one from a venv")
 
 set(FIREBASE_XCODE_TARGET_FORMAT "frameworks" CACHE STRING
                                  "Format to output, 'frameworks' or 'libraries'")

cmake/external/firestore.cmake

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 include(ExternalProject)
+include(FindPythonInterp)
 
 if(TARGET firestore)
   return()
@@ -32,6 +33,6 @@ ExternalProject_Add(
   BUILD_COMMAND ""
   INSTALL_COMMAND ""
   TEST_COMMAND ""
-  PATCH_COMMAND ${FIREBASE_PYTHON_EXECUTABLE} ${CMAKE_CURRENT_LIST_DIR}/firestore_patch.py --leveldb-version-from ${CMAKE_CURRENT_LIST_DIR}/leveldb.cmake
+  PATCH_COMMAND ${PYTHON_EXECUTABLE} ${CMAKE_CURRENT_LIST_DIR}/firestore_patch.py --leveldb-version-from ${CMAKE_CURRENT_LIST_DIR}/leveldb.cmake
   HTTP_HEADER "${EXTERNAL_PROJECT_HTTP_HEADER}"
 )