Fix test helper to prevent host environment leakage

Add env: {} to makeContext helper in tty-allocation.spec.js to prevent
buildEnvironment from falling back to process.env. This ensures test
isolation and matches the pattern used in build-exec-environment.spec.js.

Author: cursoragent

test/tty-allocation.spec.js

@@ -27,6 +27,7 @@ const makeContext = (overrides = {}) => {
     stdin: {isTTY: false, isClosed: false, ...stdin},
     stdout: {isTTY: false, columns: 80, rows: 24, ...stdout},
     stderr: {isTTY: false, ...stderr},
+    env: {},
     isNodeMode: true,
     ci: false,
     landoColorLevel: 0,