sidecar/v0.210.0+1

DI-4819: swap x86_64-apple-darwin leg to MACOS_RUNNER_INTEL (#2103)

## Summary

Follow-up to #2079. The `macos-13` leg of
`cd-build-sidecar-binaries.yaml` was disabled there (GitHub's free-tier
hosted Intel Mac pool queues indefinitely). This swaps it to
`vars.MACOS_RUNNER_INTEL` (`macos-15-intel`).

## Why this variable, not Depot

Depot (dbt-core v2's route for aarch64-apple-darwin) doesn't work here:
Depot's own docs confirm its macOS runners are Apple Silicon (M2/M4)
only, and Nuitka can't cross-compile the way dbt-core's Rust CLI builds
do. This is because the Nuitka builds embeds native CPython plus
precompiled per-arch wheels (pydantic-core, duckdb), so it needs to
actually run on Intel hardware to produce an Intel binary. However there
is still a path forward. Some of the drivers core v2 uses for adapters
are go based, and go also don't have cross compilation. For these builds
it turns out we target macos-15-intel, which is now what we do.

## Validated

`MACOS_RUNNER_INTEL` has been added as a variable on this repo.
Confirmed in two stages:

- A one-step, no-compile check scheduled and ran in 5s on genuine
`x86_64` hardware (`Darwin ... RELEASE_X86_64 x86_64`, macOS 15.7.7) --
no queueing, unlike `macos-13`.
- The full matrix then ran end-to-end via a temporary draft-PR
validation trigger (since removed): all 5 legs passed, including
`Compile & Validate mf_entry (x86_64-apple-darwin)` in 23m6s on
`macos-15-intel`.

## Caveat

GitHub has announced it's retiring Intel macOS support entirely after
the macOS 15 runner image goes away (~Fall 2027) -- this is a real fix
but not a permanent one.

Refs DI-4819