Change the way string values are interpreted from build.executable - now they must be quoted or otherwise escaped suitable for parsing by shlex.split.

Add test demonstrating how a custom launch command spec that could be passed as the script executable.

Added new class CommandSpec, which will be used for abstracting the command handling for script headers.

Move decision logic about windows/header generation closer to install_scripts, as it doesn't appear to be used elsewhere.

Skip integration tests when one of the packages under test is already installed, as the installation will fail in that case.