Fix path handling in `build.ps1`.

[jeffdav]

Some places use Join-Path, some use [IO.Path]::Combine(), and others just use string interpolation ($Path = "foo/$bar/baz" style). All three of these behave differently.

Currently CI passes in values assuming that we will concatenate a \ before appending more, but this is an unspoken convention and is not enforced. If we use [IO.Path]::Combine() and a bare drive letter is passed in (e.g. s:) this assumption breaks.

We should use Join-Path or Combine() everywhere, disallow bare drives as paths via [ValidatePattern('^(?!^[a-zA-Z]:$|^[a-zA-Z]:[^\\].*$).*$')] on the incoming params, and update the calling batch files to pass t:\ if they want stuff in the root.