ci: install e2e Go toolchain from e2e/go.mod

[joshua-temple]

Root cause

The e2e module (e2e/go.mod) declares go 1.24.0 / toolchain go1.24.4, but every workflow step that runs it pinned setup-go to go-version: '1.23'. Under GOTOOLCHAIN=local the runner refuses to fetch a newer toolchain, so go test ./... in e2e/ exits 1 immediately:

go: go.mod requires go >= 1.24.0 (running go 1.23.12; GOTOOLCHAIN=local)

This hit both the standalone E2E workflow and the Orchestrate pipeline (build-cli.yaml → E2E Tests job). The scenarios themselves are healthy — the last tag run, which happened to resolve GOTOOLCHAIN=auto, shows all of TestMultiStepScenarios passing.

Fix

Drive the installed Go straight from e2e/go.mod via go-version-file (plus cache-dependency-path: e2e/go.sum) in every step that runs the e2e module, so the toolchain always matches the module requirement regardless of GOTOOLCHAIN mode and can't drift when the go directive moves.

Steps changed:

• e2e.yaml — e2e job setup-go
• build-cli.yaml — e2e-tests job setup-go (Orchestrate path)

Root-module jobs (the build job in build-cli.yaml, plus pr.yaml, validate.yaml, release.yaml) stay on the root go.mod's 1.23 — they build ./cmd/cascade, not the e2e module.

Verification

• actionlint clean on both edited workflows (the pre-existing jobs.e2e-tests.result output warning is unrelated and present on main)
• go build ./... and (cd e2e && go build ./...) clean
• gofmt -l clean