allow setting custom OMNISHARP HOME directory

Author: filipw

src/OmniSharp.Host/Services/OmniSharpEnvironment.cs

@@ -8,12 +8,9 @@ public class OmniSharpEnvironment : IOmniSharpEnvironment
     {
         public string TargetDirectory { get; }
         public string SharedDirectory { get; }
-
         public string SolutionFilePath { get; }
-
         public int HostProcessId { get; }
         public LogLevel LogLevel { get; }
-
         public string[] AdditionalArguments { get; }
 
         public OmniSharpEnvironment(
@@ -45,9 +42,11 @@ public OmniSharpEnvironment(
             LogLevel = logLevel;
             AdditionalArguments = additionalArguments;
 
+            // First look at OMNISHARPHOME to allow users to set custom location, then
             // On Windows: %USERPROFILE%\.omnisharp\omnisharp.json
             // On Mac/Linux: ~/.omnisharp/omnisharp.json
             var root =
+                Environment.GetEnvironmentVariable("OMNISHARPHOME") ??
                 Environment.GetEnvironmentVariable("USERPROFILE") ??
                 Environment.GetEnvironmentVariable("HOME");