firestore_snappy.patch.txt: hack test to see if the windows python error can be fixed by using a temporary directory.

A previous commit, bc4a657, patched firebase-ios-sdk with firebase/firebase-ios-sdk#9662, which modified cmake builds to create a python virtualenv for each python script.

This, however, ran into a strange problem in the Windows GitHub Actions runners, where creating the virtualenv failed when it attempted to call ensurepip.

The suspected root cause is a MAX_PATH length violation.

To test out this hypothesis, this commit modifies the FirebaseSetupPythonInterpreter() to create the Python virtualenv in a temporary directory instead of a well-defined subdirectory of cmake's binary directory.

This should have a shorter path length for the virtualenv. If this works, then possibly we will need to provide a way to override the base directory for the pyvenv's.

Here is the actual failure from https://github.com/firebase/firebase-cpp-sdk/runs/6083486670

```
-- Found PythonInterp: C:/ProgramData/chocolatey/bin/python2.7.exe (found suitable version "2.7.9", minimum required is "2.7")
-- Found Python3: C:/hostedtoolcache/windows/Python/3.10.4/x64/python3.exe (found version "3.10.4") found components: Interpreter
-- FirebaseSetupPythonInterpreter(FirestoreProtos): Creating Python virtualenv in D:/a/firebase-cpp-sdk/firebase-cpp-sdk/ta/admob/it/bin/external/src/firestore-build/Firestore/Protos/pyvenv/FirestoreProtos using C:/hostedtoolcache/windows/Python/3.10.4/x64/python3.exe
Error: Command '['D:\\a\\firebase-cpp-sdk\\firebase-cpp-sdk\\ta\\admob\\it\\bin\\external\\src\\firestore-build\\Firestore\\Protos\\pyvenv\\FirestoreProtos\\Scripts\\python3.exe', '-Im', 'ensurepip', '--upgrade', '--default-pip']' returned non-zero exit status 3221225781.
CMake Error at bin/external/src/firestore/cmake/python_setup.cmake:133 (message):
  Failed to create a Python virtualenv in
  D:/a/firebase-cpp-sdk/firebase-cpp-sdk/ta/admob/it/bin/external/src/firestore-build/Firestore/Protos/pyvenv/FirestoreProtos
  using C:/hostedtoolcache/windows/Python/3.10.4/x64/python3.exe
Call Stack (most recent call first):
  bin/external/src/firestore/Firestore/Protos/CMakeLists.txt:16 (FirebaseSetupPythonInterpreter)
```

Author: dconeybe

cmake/external/firestore_snappy.patch.txt

@@ -887,3 +887,39 @@ index 000000000..bdb7b9f6a
 +
 +  set("${ARG_OUTVAR}" "${PYTHON_EXECUTABLE}" PARENT_SCOPE)
 +endfunction(FirebaseSetupPythonInterpreter)
+diff --git a/cmake/python_setup.cmake b/cmake/python_setup.cmake
+index bdb7b9f6a..152744fab 100644
+--- a/cmake/python_setup.cmake
++++ b/cmake/python_setup.cmake
+@@ -91,12 +91,30 @@ function(FirebaseSetupPythonInterpreter)
+     "The Python interpreter on the host system to use"
+   )
+ 
++  # Create a temporary directory into which to install the virtualenv
++  execute_process(
++    COMMAND
++      "${FIREBASE_PYTHON_HOST_EXECUTABLE}"
++      -c
++      "import tempfile; print(tempfile.mkdtemp('FirebaseSetupPythonInterpreter', 'pyvenv'))"
++    RESULT_VARIABLE
++      FIREBASE_PYVENV_MKDTEMP_RESULT
++    OUTPUT_VARIABLE
++      FIREBASE_PYVENV_MKDTEMP_OUTPUT
++    OUTPUT_STRIP_TRAILING_WHITESPACE
++  )
++  if(NOT FIREBASE_PYVENV_MKDTEMP_RESULT EQUAL 0)
++    message(FATAL_ERROR
++      "Failed to create a temporary directory for the Python virtualenv "
++      "using ${FIREBASE_PYTHON_HOST_EXECUTABLE}")
++  endif()
++
+   # Check if the virtualenv is already up-to-date by examining the contents of
+   # its stamp files. The stamp files store the path of the host Python
+   # interpreter and the dependencies that were installed by pip. If both of
+   # these files exist and contain the same Python interpreter and dependencies
+   # then just re-use the virtualenv; otherwise, re-create it.
+-  set(PYVENV_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/pyvenv/${ARG_KEY}")
++  set(PYVENV_DIRECTORY "${FIREBASE_PYVENV_MKDTEMP_OUTPUT}/${ARG_KEY}")
+   set(STAMP_FILE1 "${PYVENV_DIRECTORY}/cmake_firebase_python_stamp1.txt")
+   set(STAMP_FILE2 "${PYVENV_DIRECTORY}/cmake_firebase_python_stamp2.txt")
+