fix(nextjs): fix inlined workspace root in .nx-helpers (#27136)

This PR fixes an issue where our `withNx` function for Next.js is
compiled with inlined `workspaceRoot` (to support container
environments). On Windows, we write the `.nx-helpers/with-nx.js` with
the following:

```js
workspaceRoot: 'C:\Users\user\projects\app',
```

The `\u` character result in a Node error: `Invalid Unicode escape
sequence`.

The fix is to escape `\` as `\\`, so when the file is written, the path
is valid and does not cause unicode errors.

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## Current Behavior
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## Expected Behavior
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## Related Issue(s)
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Fixes #18824

packages/next/src/executors/build/lib/create-next-config-file.ts

@@ -124,7 +124,11 @@ export function getWithNxContent(
         index: getWithNxContextDeclaration.getStart(withNxSource),
         text: stripIndents`function getWithNxContext() {
           return {
-            workspaceRoot: '${workspaceRoot}',
+            workspaceRoot: '${
+              // For Windows, paths like C:\Users\foo\bar need to be written as C:\\Users\\foo\\bar,
+              // or else when the file is read back, the single "\" will be treated as an escape character.
+              workspaceRoot.replaceAll('\\', '\\\\')
+            }',
             libsDir: '${workspaceLayout().libsDir}'
           }
         }`,