ci(homebrew): add HOMEBREW_NO_SANDBOX_LINUX=1 (temporary fix)

[stasadev]

The Issue

• setup-homebrew does not set up the linux sandbox Homebrew/actions#865

Changes in Homebrew 6.0.0 and Homebrew/actions/setup-homebrew is not updated for this.

https://github.com/ddev/github-action-add-on-test/actions/runs/27355373227/job/80828105647?pr=64

Error: Bubblewrap is installed but cannot create a rootless sandbox.

Homebrew's Linux sandbox requires rootless Bubblewrap and unprivileged
user namespaces. Check and update this system configuration:
  sudo sysctl -w kernel.unprivileged_userns_clone=1
    Allows unprivileged processes to create user namespaces. Rootless
    Bubblewrap needs this to isolate builds without elevated privileges.
  sudo sysctl -w user.max_user_namespaces=28633
    Allows each user to allocate enough user namespaces. A zero or low
    limit can prevent Bubblewrap from creating its sandbox.
  sudo sysctl -w kernel.apparmor_restrict_unprivileged_userns=0 || true
    Allows unprivileged user namespaces on AppArmor-enabled systems
    that restrict them by default. Older kernels may not provide this
    setting.

As a final workaround, disable the Linux sandbox:
  export HOMEBREW_NO_SANDBOX_LINUX=1

How This PR Solves The Issue

Adds HOMEBREW_NO_SANDBOX_LINUX=1, I'm going to remove it once the upstream is updated.

Manual Testing Instructions

Automated Testing Overview

Release/Deployment Notes

Don't create releases manually. Follow the release process documentation when making a new release.