Resolve host architecture in legacy toolregistry Darwin install scripts

[rootp1]

What this PR does:

Parameterizes the legacy (v0) piped tool registry's Darwin (and Linux) install scripts for kubectl, kustomize, helm, and terraform with the host architecture (runtime.GOARCH) instead of a hardcoded amd64, matching the pattern already used by the pipedv1 tool registries. Adds a focused test asserting the rendered Darwin install scripts reference the host architecture rather than a hardcoded value.

Why we need it:

On darwin/arm64 (Apple Silicon) hosts, the legacy tool registry downloaded darwin/amd64 binaries, which cannot execute natively (fork/exec ...: bad CPU type in executable), breaking local development and any test relying on these tools (e.g. pkg/app/piped/platformprovider/kubernetes).

Which issue(s) this PR fixes:

Fixes #7005

Does this PR introduce a user-facing change?:

• How are users affected by this change: Developers running piped v0 (legacy) tooling on darwin/arm64 now get architecture-correct kubectl/kustomize/helm/terraform binaries instead of incompatible amd64 ones. No behavior change on amd64 hosts.
• Is this breaking change: No
• How to migrate (if breaking change): Not applicable

Verification

• go build ./... — passed
• go vet ./pkg/app/piped/toolregistry/... — passed
• gofmt -l on changed files — passed (no output)
• go test ./pkg/app/piped/toolregistry/... — passed
• go test ./pkg/app/piped/platformprovider/kubernetes/... (with tool cache cleared, real downloads) — TestTemplateLocalChart_WithNamespace and TestKustomizeTemplate_WithHelm (helm-based paths) now pass on darwin/arm64, where they previously failed with bad CPU type in executable. TestKustomizeTemplate still fails, but for an unrelated reason: the pinned default kustomize version (3.8.1) never published a darwin_arm64 release artifact upstream (confirmed via direct HTTP HEAD, 404), so no code-level fix can download it. This is a pre-existing upstream artifact gap for that specific pinned version, not a regression from this change.
• go vet ./... — pre-existing unrelated lock-copy warnings across the repo (not touched by this change); no new warnings introduced by this diff.
• make check (full target, requires Docker-based golangci-lint, web build, and envtest/Docker-backed integration tests) — not run: Docker and golangci-lint are not available in this environment.
• ./hack/ensure-dco.sh — reported an unrelated pre-existing failure because it diffs against the fork's stale origin/master (13 commits behind upstream/master); the actual commit on this branch carries a proper Signed-off-by trailer (verified via git log).

Scope

• Only pkg/app/piped/toolregistry/install.go, pkg/app/piped/toolregistry/tool_darwin.go, and the new pkg/app/piped/toolregistry/tool_darwin_test.go were changed. No unrelated files were modified.