Pass along FIREBASE_PYTHON_HOST_EXECUTABLE for leveldb.cmake

[dconeybe]

Fix a bug where setting up the leveldb build in cmake would sometimes use an incorrect python interpreter when patching its source code. This happened because the leveldb build is invoked in a child cmake process, to which the cache variables are explicitly passed in via cmake's -D command-line argument.

The FIREBASE_PYTHON_HOST_EXECUTABLE cache variable was introduced in #9662, and the leveldb build was modified in #9596 to use it; however, it must have gone unnoticed that it was ignoring the FIREBASE_PYTHON_HOST_EXECUTABLE cache variable. This PR fixes this.

Here is where this bug was discovered: firebase/firebase-cpp-sdk#967 (comment)

[google-oss-bot]

Coverage Report ¹

Affected Products

• FirebaseFirestore-iOS-FirebaseFirestore.framework

  Filename	Base (55a8146)	Merge (6368ef0)	Diff
  leveldb_key.cc	98.33%	98.14%	-0.20%
  write_stream.cc	91.55%	94.37%	+2.82%

Test Logs

• Base (55a8146): https://github.com/firebase/firebase-ios-sdk/actions/runs/2392715531
• Merge (6368ef0): https://github.com/firebase/firebase-ios-sdk/actions/runs/2392953780

1. https://storage.googleapis.com/firebase-sdk-metric-reports/JHAEYBMod0.html