[test] Use NEXT_TEST_CI when forking test in CI (#79354)

test/development/experimental-https-server/https-server.generated-key.test.ts

@@ -8,11 +8,11 @@ describe('experimental-https-server (generated certificate)', () => {
     startCommand: `pnpm next ${
       shouldRunTurboDevTest() ? 'dev --turbo' : 'dev'
     } --experimental-https`,
-    skipStart: !process.env.CI,
+    skipStart: !process.env.NEXT_TEST_CI,
   })
   if (skipped) return
 
-  if (!process.env.CI) {
+  if (!process.env.NEXT_TEST_CI) {
     console.warn('only runs on CI as it requires administrator privileges')
     it('only runs on CI as it requires administrator privileges', () => {})
     return