Force the windows workflow to use a specific python path

[a-maurice]

Description

Provide details of the change, and generalize the change in the PR title above.

Firestore is having a problem with python, as it finds multiple versions. This can be worked around by setting FIREBASE_PYTHON_HOST_EXECUTABLE, so for now force that to be the version of python we are installing.

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Testing

Describe how you've tested these changes.

Ran this: https://github.com/firebase/firebase-unity-sdk/actions/runs/2210433023

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Type of Change

Place an x the applicable box:

• Bug fix. Add the issue # below if applicable.
• New feature. A non-breaking change which adds functionality.
• Other, such as a build process or documentation change.

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[github-actions]

❌  Integration test FAILED

Requested by @a-maurice on commit f00f38f
Last updated: Fri Apr 22 20:25 PDT 2022
View integration test log & download artifacts

Failures	Configs
firestore	[TEST] [FAILURE] [2019] [macos] [1/5 Platform(s): Android] [1/2 Test Device(s): android_target]
messaging	[TEST] [ERROR] [2019] [macos] [2/5 Platform(s): iOS Android] [All 2 Test Device(s)]
storage	[BUILD] [ERROR] [2019] [macos] [All 3 Platform(s)]