Fix conda install test: use local channel, isolate install dir from repo workspace

[Copilot]

Replace the prefix.dev publish-then-install approach with a direct local-channel install, and move the install directory outside the repo to prevent pixi from walking up to the parent workspace.

Changes

• Removed details, build_conda, publish_prefix_dev, and test_conda_install jobs — no artifacts uploaded, no secrets required
• New build_and_install matrix job (ubuntu/windows/macos): builds with pixi build, then installs from the resulting output/ directory as a file:// conda channel in a fully fresh pixi project
• Install directory moved to $RUNNER_TEMP to ensure pixi cannot walk up the directory tree and inherit the repo workspace's channel list (sonic-forge) or other config — guaranteeing the environment mirrors a user starting from scratch with only conda-forge

LOCAL_CHANNEL="$(pwd)/output"
mkdir "$RUNNER_TEMP/soundscapy-install-test"
cd "$RUNNER_TEMP/soundscapy-install-test"
pixi init
pixi project channel add "file://${LOCAL_CHANNEL}"
pixi add soundscapy