[ci] [R-package] use 'pixi' for macOS Intel job, cut some macos-15-intel jobs #7414

Description

fixes CI 😫

Problem 1: unable to brew install qpdf on x86_64 macOS

Fixes #7413

Apple is planning to stop supporting older Macs with Intel chips over the next few years, and repackagers like Homebrew are already starting to drop support (see #7412 for details).

I think LightGBM should try to support it for as long as possible, but at a minimum we need to support it in the R package for as long as CRAN does (https://cran.r-project.org/web/checks/check_flavors.html).

To help fix the immediate CI failures and hopefully make it easier to maintain this support, this proposes the following changes:

• use a locked pixi environment for R CI on Intel Macs


• move Python-specific dependencies out of the default pixi environment


• get BasicTex from a LaTeX mirror, instead of Homebrew


• drop macos-15-intel CI jobs that do not produce release artifacts (this should help with #7292 too, since we face a low concurrency limit for those)


• group remaining brew installs of packages into fewer installs

Problem 2: Python wheel builds failing on Windows

Fixes #7415

• build Windows wheels with --no-isolation, to avoid build creating a venv