build.sh default configuration argument creates unexpected behavior

### What You Are Seeing?
On Mac I changed:

var configuration = Argument("configuration", "Release");

to

var configuration = Argument("configuration", "Debug");

Running the script ("./build.sh"), I see that argument value is Release when I provided a default of Debug:


### What is Expected?
I expect to see the new default value for the configuration argument be set to Debug per the change to the script.

When I run this on Windows the same way ".\build.ps1", I see the configuration argument value set to Debug.

### What version of Cake are you using?
0.27.0

### Are you running on a 32 or 64 bit system?
64

### What environment are you running on? Windows? Linux? Mac?
Mostly Mac but both Mac and Windows 10 in this case.

### Are you running on a CI Server? If so, which one?
No, local only.

### How Did You Get This To Happen? (Steps to Reproduce)
1. Default Cake example repo was cloned on my Mac

2. On Mac in VS Code in build.cake I changed

var configuration = Argument("configuration", "Release");

to

var configuration = Argument("configuration", "Debug");

and saved the script.

3. In VS Code's integrated terminal on Mac, I invoked the script with only "./build.sh"

4. Because I specified no configuration argument when invoking build.sh, I expected the Cake script to see no configuration argument value and to take the default value I provided of Debug (not overriding it with a default in build.sh). That expectation is reinforced with build.ps1 which does respect the fact that I didn't pass in a configuration argument so it uses my default of Debug.

I expect build.sh to do something similar to build.ps1:

if ($Configuration) { $cakeArguments += "-configuration=$Configuration" }

### Files
[build-files.zip](https://github.com/cake-build/cake/files/1951945/build-files.zip)

Start with build.sh and compare its configuration handling with build.ps1, then review the configuration argument in build.cake. Run ./build.sh without a configuration argument and confirm that the Cake script receives Debug after the script default is changed, matching the Windows behavior.

Written by the indexing model from the issue text.