ci: drop hard-coded julia-arch x64 so macOS tests run on native arm64

[ChrisRackauckas-Claude]

Problem

The default branch (main) CI was red on all three macOS test jobs:

• Tests (1, macOS-latest)
• Tests (lts, macOS-latest)
• Tests (pre, macOS-latest)

Each died in the setup-julia step before any Julia ran:

[setup-julia] x64 arch has been requested on a macOS runner that has an
arm64 (Apple Silicon) architecture. You may have meant to use the
"aarch64" arch instead (or "default" or left it unspecified ...).

Root cause

macOS-latest is now Apple Silicon (arm64). The Tests caller passed
julia-arch: "x64" to the centralized reusable workflow
(SciML/.github/.github/workflows/tests.yml@v1), which forwards it as:

arch: "${{ inputs.julia-arch || runner.arch }}"

The hard-coded x64 overrode the runner.arch fallback, producing an
invalid x64-on-arm64 request on macOS-latest. The ubuntu/windows jobs were
unaffected; only the macOS jobs failed.

Fix

Remove the julia-arch: "x64" line from Tests.yml. The reusable
workflow then falls back to runner.arch, which resolves to the runner's
native architecture (x64 on ubuntu/windows, arm64 on macOS-latest).

Local verification

julia +1.12 --project=. Pkg.test() (the 1 channel is 1.12):

Test Summary:       | Pass  Total   Time
LightweightStats.jl |  455    455  52.3s
     Testing LightweightStats tests passed

The test suite is healthy; the CI failure was purely the arch
misconfiguration, which never let the tests run.

Out of scope (separate failure)

The Documentation check is also red, but for an unrelated infra reason:
the docs build succeeds, then the gh-pages deploy fails with
remote: Permission to SciML/LightweightStats.jl.git denied to github-actions[bot] (403). DOCUMENTER_KEY is empty and the
github-actions[bot] token lacks push permission. That requires a repo
secret / token-permission change by a maintainer and cannot be fixed by
editing repo code, so it is not addressed here.

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Please ignore until reviewed by @ChrisRackauckas