Support environment variables in paths

Paths should support environment variables (on Mono and Windows).

Segments surrounded by % tokens are not technically valid paths (on Windows), but we’re going to make an exception for this. This means that we should no longer throw an exception when creating an illegal path, but mark the path as invalid. The $ token is valid on *nix systems, so a path like that will still be considered valid.

A path containing environment variables will be considered invalid. Tools and script aliases that work with paths will have to take this into consideration and act accordingly.

Path

• bool Path.ContainsEnvironmentVariables()
• bool IsValidPath { get; }

FilePath

• FilePath ExpandEnvironmentVariables(ICakeEnvironment environment)

DirectoryPath

• DirectoryPath ExpandEnvironmentVariables(ICakeEnvironment environment)

[rofr]

% is a valid file name char on *nix. And usually env vars are prefixed with $, at least in shell scripts. How about just support for interpolation of local variable names?

[patriksvensson]

Superseded by #2312