C#/C++: Use CODEQL_EXTRACTOR_CPP_ROOT in autobuild

Left this out earlier because I thought it needed to point to the C#
extractor root even in C++ mode, but it looks like it isn't yet used in
C++ mode.

Author: Robert Marsh

csharp/autobuilder/Semmle.Autobuild.Tests/BuildScripts.cs

@@ -344,7 +344,7 @@ Autobuilder CreateAutoBuilder(string lgtmLanguage, bool isWindows,
             Actions.GetEnvironmentVariable[$"CODEQL_AUTOBUILDER_{codeqlUpperLanguage}_NO_INDEXING"] = "false";
             Actions.GetEnvironmentVariable[$"CODEQL_EXTRACTOR_{codeqlUpperLanguage}_TRAP_DIR"] = "";
             Actions.GetEnvironmentVariable[$"CODEQL_EXTRACTOR_{codeqlUpperLanguage}_SOURCE_ARCHIVE_DIR"] = "";
-            Actions.GetEnvironmentVariable["CODEQL_EXTRACTOR_CSHARP_ROOT"] = @"C:\codeql\csharp";
+            Actions.GetEnvironmentVariable[$"CODEQL_EXTRACTOR_{codeqlUpperLanguage}_ROOT"] = $@"C:\codeql\{lgtmLanguage}";
             Actions.GetEnvironmentVariable["CODEQL_JAVA_HOME"] = @"C:\codeql\tools\java";
             Actions.GetEnvironmentVariable["SEMMLE_DIST"] = @"C:\odasa";
             Actions.GetEnvironmentVariable["SEMMLE_JAVA_HOME"] = @"C:\odasa\tools\java";

csharp/autobuilder/Semmle.Autobuild/Autobuilder.cs

@@ -184,7 +184,7 @@ public Autobuilder(IBuildActions actions, AutobuildOptions options)
                 return ret ?? new List<IProjectOrSolution>();
             });
 
-            CodeQLExtractorCSharpRoot = Actions.GetEnvironmentVariable("CODEQL_EXTRACTOR_CSHARP_ROOT");
+            CodeQLExtractorLangRoot = Actions.GetEnvironmentVariable($"CODEQL_EXTRACTOR_{this.Options.Language.UpperCaseName}_ROOT");
             SemmleDist = Actions.GetEnvironmentVariable("SEMMLE_DIST");
             SemmlePlatformTools = Actions.GetEnvironmentVariable("SEMMLE_PLATFORM_TOOLS");
 
@@ -194,9 +194,9 @@ public Autobuilder(IBuildActions actions, AutobuildOptions options)
                 throw new InvalidEnvironmentException("The environment variable CODEQL_JAVA_HOME or SEMMLE_JAVA_HOME has not been set.");
 
             Distribution =
-                CodeQLExtractorCSharpRoot ??
+                CodeQLExtractorLangRoot ??
                 SemmleDist ??
-                throw new InvalidEnvironmentException("The environment variable CODEQL_EXTRACTOR_CSHARP_ROOT or SEMMLE_DIST has not been set.");
+                throw new InvalidEnvironmentException($"The environment variable CODEQL_EXTRACTOR_{this.Options.Language.UpperCaseName}_ROOT or SEMMLE_DIST has not been set.");
 
             TrapDir =
                 Actions.GetEnvironmentVariable($"CODEQL_EXTRACTOR_{this.Options.Language.UpperCaseName}_TRAP_DIR") ??
@@ -397,9 +397,9 @@ BuildScript AutobuildFailure() =>
                 });
 
         /// <summary>
-        /// Value of CODEQL_EXTRACTOR_CSHARP_ROOT environment variable.
+        /// Value of CODEQL_EXTRACTOR_<LANG>_ROOT environment variable.
         /// </summary>
-        private string? CodeQLExtractorCSharpRoot { get; }
+        private string? CodeQLExtractorLangRoot { get; }
 
         /// <summary>
         /// Value of SEMMLE_DIST environment variable.