Change env variable retrieval to work on unix

Author: TheOnlyWei

src/Az.BootStrapper/Module/Az.Bootstrapper.psm1

@@ -169,9 +169,11 @@ function Get-AzProfileMap
   [CmdletBinding()]
   param([Switch]$Update)
 
-  $azBootStrapperLocalProfile = [System.Environment]::GetEnvironmentVariable("AzBootStrapperLocalProfile","Machine")
+  # Must use Get-Item and not [Environment]::GetEnvironmentVariable since the latter doesn't work on Unix systems.
+  $azBootStrapperLocalProfile = Get-Item -Path Env:AzBootStrapperLocalProfile -ErrorAction SilentlyContinue
   if ($azBootStrapperLocalProfile)
   {
+    $azBootStrapperLocalProfile = $azBootStrapperLocalProfile.Value
     $scriptBlock = {
         Get-Content -Path $azBootStrapperLocalProfile -ErrorAction stop | ConvertFrom-Json 
     }

src/Az.BootStrapper/readme.md

@@ -20,7 +20,7 @@ Install-AzProfile -Profile <PROFILE_VERSION>
 - To install local profile, set the environment variable AzBootStrapperLocalProfile with the full path to the local profile on your system.
 
 ```
-[Environment]::SetEnvironmentVariable("AzBootStrapperLocalProfile", <PATH_TO_PROFILE_JSON>, [System.EnvironmentVariableTarget]::Machine)
+Set-Item -Path Env:AzBootStrapperLocalProfile -Value <FULL_PATH_TO_PROFILE_JSON>
 Set-BootStrapRepo -Repo <REPOSITORY>
 Install-AzProfile -Profile <PROFILE_VERSION>
 ```