Build.ps1 doesn't play nice with an existing 'tools' directory

If a `tools` directory already exists, then the current `build.ps1` appears to want to delete all the contents. This is not very neighbourly of it (especially as it isn't that unusual for a solution to have a tools directory).

Ideally, build.ps1 should:
1. warn if there's an existing tools directory and confirm with the user before splatting the contents
2. respect the values configured in a cake.config file that optionally override what actual directory to use instead of the default 'tools'.

Start by reading build.ps1 and reproducing the behavior with an existing tools directory. Then inspect the cake.config handling to determine how an alternate tools path is configured. Done means existing contents are protected by a confirmation prompt and the configured directory is respected.

Written by the indexing model from the issue text.