Pass in -j N to cmake to speed up desktop testapp build.

[jonsimantov]

Description

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

Pass in "-j <number_of_cpus>" to cmake --build when building desktop testapps.

This should speed up the desktop testapp build. Linux improves from ~50min to ~35min. MacOS improves from ~140min to ~90min. Windows does not change (probably ignores the -j parameter).

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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.

Run integration tests in this PR.

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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 with FLAKINESS (succeeded after retry)

Requested by @jonsimantov on commit 18f20ad
Last updated: Sat May 7 02:52 PDT 2022
View integration test log & download artifacts

Failures	Configs
database	[TEST] [FLAKINESS] [Android] [1/3 os: windows] [1/2 android_device: android_target] (1 failed tests)   FirebaseDatabaseTest.TestInfoConnected

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

[jonsimantov]

Thanks, I'm also going to see if building the iOS SDK one architecture at a time, but using -j on each one, saves time over building all of the architectures at once. Will revert that if it doesn't have any positive effect.