Update/minor fixes gtest

[LeStarch]

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Has Unit Tests (y/n)
Builds Without Errors (y/n)
Unit Tests Pass (y/n)
Documentation Included (y/n)

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Change Description

Moves GTest clone and build into build cache to prevent contention. Adds prevent_prescan guard to stop FPP prescan builds from working in a directory.

[LeStarch]

Surpassed by #2021 .