docs: document fluid-build env vars and in-repo build-tools linking

[tylerbutler]

Description

Documentation-only updates for fluid-build configuration.

• build-tools/packages/build-tools/README.md — Refresh the environment variables section: rename the table's Task column to Value and document each variable's expected value, add the previously undocumented FLUID_BUILD_EXTENDED_METRICS_DISPLAY and FLUID_BUILD_TEST_RANDOM_ORDER variables, and clarify the surrounding prose.
• pnpm-workspace.yaml — Add commented-out overrides entries (with a link to the build-tools README) that contributors can uncomment to link the in-repo copies of the build tools for local testing.

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[Copilot]

Pull request overview

Documentation-only updates to make fluid-build configuration more discoverable and easier to test locally by clarifying supported environment variables and documenting an opt-in pnpm override workflow for linking in-repo build-tools packages.

Changes:

• Refined the fluid-build environment variables section to document expected values and add two previously undocumented variables.
• Added commented pnpm override entries that contributors can uncomment to link in-repo build-tools packages for local testing.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
pnpm-workspace.yaml	Adds commented-out overrides entries to optionally link in-repo build-tools packages for local validation.
build-tools/packages/build-tools/README.md	Updates fluid-build env var documentation (expected values, new vars, clarified prose).