build(bundle-size-tests): Update fluid build task deps

[alexvy86]

Description

Updating fluid build task dependencies per Abram's suggestion so building the bundle-size-tests package locally results in a state where the tests can actually run and succeed.

The webpack task already depends on build:esnext in the base fluid build config so the entry here was somewhat redundant, but also it prevented the build:test -> webpack dependency because it caused circular task dependencies.

I confirmed this now works locally; after running pnpm build, pnpm test successfully runs the tests and they pass. Of note, pnpm build:test is not enough to make that work because that just runs the build:test npm script which does not cause fluid-build to run. On the other hand, the build npm script invokes fluid-build and all the dependencies work correctly.

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[alexvy86]

Closing in favor of #24162