Move local_engine builds (or a subset) to post-submit only #195645

local_engine runs gn/ninja for all et-possible local engine builds. We want to detect local engine development regressions. But it also takes up 11% of all Mac bot-hours (12% of presubmit Mac bot-hours, 8.9% of post-submit Mac bot-hours) which is quite a lot of valuable Mac CI resources for something non-user-facing. Just macos/wasm_debug_unopt and macos/wasm_release together take up 2.5% of all Mac bot-hours since it's no-goma, etc

This builder was suppressed for a very long time and wasn't running in presubmit. Some options:

1. Move the whole local_engine builder to post-submit only
2. Move all the sub-builds to post-submit except the most common variant for each platform (debug unopt).
3. Build in presubmit with a runIf like engine/src/flutter/tools/**, engine/src/flutter/ci/builders/local_engine.json, **/BUILD.gn, **/*.gni

Additionally, _release versions of the local engine are not often used. Consider building it even less often, say every 5 commits in post-submit?