GH2440: Add EnvironmentVariable<T> alias

[gitfool]

Fixes #2440

[devlead]

LGTM 👍

[devlead]

@gitfool your changes have been merged, thanks for your contribution 👍

[gitfool]

@devlead I hit an issue when trying to use Cake after this merge - I've just discovered there's another implementation of this method in Cake.Incubator/EnvironmentExtensions.cs#L64 that causes an error when compiling the build script:

Compiling build script...
(3312,10): error CS0111: Type 'Submission#0' already defines a member called 'EnvironmentVariable' with the same parameter types
Error: Cake.Core.CakeException: Error(s) occurred when compiling build script:
(3312,10): error CS0111: Type 'Submission#0' already defines a member called 'EnvironmentVariable' with the same parameter types
   at Cake.Scripting.Roslyn.RoslynScriptSession.Execute(Script script)
   at Cake.Core.Scripting.ScriptRunner.Run(IScriptHost host, FilePath scriptPath, IDictionary`2 arguments)
   at Cake.Commands.BuildCommand.Execute(CakeOptions options)
   at Cake.CakeApplication.Run(CakeOptions options)
   at Cake.Program.Main()

I don't know how I missed this since I always use Cake.Incubator, but I guess great minds think alike? 😊

Anyway, the question is how to proceed from here? We can either revert, or move the other method over from Cake.Incubator and then remove them from there. Personally, I think it would be nice to keep this in Cake itself without needing another dependency. What do you reckon?

[devlead]

@gitfool this was bound to happen sooner or later.

Raised issue #2487 to track this.