Return all variables with matching prefix

[fourpastmidnight]

So, I was wondering, what if there happened to be an environment variable already named <PREFIX><Buildvar>, e.g., BHBuildSystem? What would happen? would BuildHelpers overwrite it? None of the function documentation mentioned what would happen, and I couldn't overtly see that anything would be overwritten--in fact, it looked like a check was made. But, still, what would happen? So I ran a test:

PS C:\> ${ENV:BUILD_BuildSystem} = 'Local'
PS C:\> Set-BuildEnvironment -VariableNamePrefix 'BUILD_' -BuildOutput ./out -PassThru

Name                          Value
----                          -----
BUILD_ProjectPath             C:\MyProject\src\MyProject
BUILD_BranchName              build-enhancements
BUILD_CommitMessage           build: Update build script
BUILD_CommitHash              123abc...
BUILD_BuildNumber             0
BUILD_ProjectName             MyProject
BUILD_PSModuleManifest        C:\MyProject\src\MyProject.psd1
BUILD_ModulePath              C:\MyProject\src
BUILD_BuildOutput             C:\MyProject\src\MyProject\out

What happened to the BUILD_BuildSystem variable? It wasn't emitted. Why not? It's still there, of course:

PS C:\> Get-Item $ENV:\BUILD_BuildSystem
Local

It would be better if BuildHelpers returned all environment variables that it would set, even if BuildHelpers did not itself actually set the variable, e.g., if a variable that BuildHelpers would normally set already has a value in the environment, simply return that variable.