Set Turbopack env var for internal modules (#77902)

Sets `process.env.TURBOPACK` globally when running with Turbopack. This
is relied upon in some cases with internals, specifically with files
that are not bundled in e.g. Pages Router. It should not be used by
application code.

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---------

Co-authored-by: Niklas Mischkulnig <[email protected]>

Author: timneutkens

turbopack/crates/turbopack-ecmascript-runtime/js/src/nodejs/runtime.ts

@@ -27,6 +27,8 @@ type SourceInfo =
       parentId: ModuleId;
     };
 
+process.env.TURBOPACK = '1';
+
 function stringifySourceInfo(source: SourceInfo): string {
   switch (source.type) {
     case SourceType.Runtime:

turbopack/crates/turbopack-tests/tests/snapshot/runtime/default_build_runtime/output/[turbopack]_runtime.js

@@ -429,6 +429,7 @@ var SourceType = /*#__PURE__*/ function(SourceType) {
    */ SourceType[SourceType["Parent"] = 1] = "Parent";
     return SourceType;
 }(SourceType || {});
+process.env.TURBOPACK = '1';
 function stringifySourceInfo(source) {
     switch(source.type){
         case 0: