Remove vcpkg from testapp builder

[a-maurice]

Description

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

vcpkg has been having errors finding the toolchain files, and should not be necessary for the build, so remove it from the testapp builder logic.

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Testing

Describe how you've tested these changes. Link any manually triggered Integration tests or CPP binary SDK Packaging Github Action workflows, if applicable.

https://github.com/firebase/firebase-cpp-sdk/actions/runs/3099919285

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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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Notes

• Bug fixes and feature changes require an update to the Release Notes section of release_build_files/readme.md.
• Read the contribution guidelines CONTRIBUTING.md.
• Changes to the public API require an internal API review. If you'd like to help us make Firebase APIs better, please propose your change in a feature request so that we can discuss it together.

[github-actions]

❌  Integration test FAILED

Requested by @a-maurice on commit 5e79c61
Last updated: Wed Sep 21 16:22 PDT 2022
View integration test log & download artifacts

Failures	Configs
missing_log	[TEST] [ERROR] [Android] [1/3 os: ubuntu] [1/2 android_device: android_target]

Add flaky tests to go/fpl-cpp-flake-tracker