Bootstrap: environmental sanitisation, package install robustness

[tzarc]

Description

Bootstrap script:

• Sanitise the inherited environment on startup: force LC_ALL=C, drop out of any active Python venv, wipe PYTHON* variables (keeping PYTHON_TARGET_VERSION), and clear conda/pip/uv overrides that would affect dependency resolution or builds. Proxy, TLS, and index/mirror variables survive, since corporate networks need them.
• Pass --no-project to uv run when determining QMK_DISTRIB_DIR, so uv ignores any pyproject.toml in the invoking directory.
• Homebrew has dropped Intel macOS to tier 3 and no longer builds bottles for it (see Intel macOS bottles missing Homebrew/homebrew-core#301207). brew install/brew upgrade now run per-formula and retry with --build-from-source when no bottle exists. Intel machines get a warning before the pre-install countdown that source builds can take a long time.

CI workflow:

• Gentoo containers fall back through emaint sync -a || emerge --sync --quiet || emerge-webrsync -q, since rsync-based sync is unreliable from GitHub runners (no IPv6 routing, flaky mirrors) and webrsync has been solid.
• The Linux prerequisite retry loop now &&-chains each distro's metadata sync and package install, so a failed sync no longer counts as a successful attempt, and the step fails outright after 10 failed attempts.
• macOS runners retry brew update up to 10 times (with brew update-reset between attempts) before running the bootstrap script, and stop at the first success.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[tzarc]

Fixes at least one failure mode: https://discord.com/channels/440868230475677696/473506116718952450/1544263017632636969