No executable found for scripts when using --symlink-install and install(PROGRAMS)

Redirected from [colcon-core #638](https://github.com/colcon/colcon-core/issues/638)

**TLDR**
On Linux systems:
Exectuables installed without `--symlink-install` gain executing permissions fine.
Exectuables installed with `--symlink-install` do not gain executing permission because symbolic links do not have their own permissions.

@cottsay suggested one two things:
_1. Detect this scenario and fall back to the non-symlink behavior while possibly displaying a warning
2. Refuse to do anything and demand that the developer make the original script executable so that we can proceed with the symlink_

I would strongly vote for solution 1. as the loss of permissions may easily be coming from an automation pipeline or such. Refusal would be terrible in a scenario like that.