Fix Homebrew "Broken pipe" failures during package install on WSL/Debian

[Copilot]

On WSL/Debian, installs failed with Error: Broken pipe. Homebrew's concurrent/threaded output can receive SIGPIPE when its stdout is a command-substitution pipe ($(...)) rather than a TTY or regular file, aborting installs (e.g. microsoft/apm/apm).

Changes

• run_onchange_10-install-packages.sh.tmpl — brew_install_quiet, brew_install_cask_quiet, and mas_install_quiet now redirect install output to a mktemp file instead of capturing it through a pipe; on failure the file is streamed into log_data. The "only surface output on failure" behavior is unchanged.
• test-chezmoi-scripts.bats — added an assertion that output is captured via file redirection and no longer via output=$(brew install ...).

brew_log=$(mktemp)
if brew install "$pkg" >"$brew_log" 2>&1; then
    log_result "Installed $pkg"
else
    log_warn "Failed to install $pkg"
    log_data WARN "brew output" <"$brew_log"
fi
rm -f "$brew_log"

Writing to a regular file removes the pipe reader that could close early, eliminating the SIGPIPE.