[antlir2][install/tests] add true.c to a buck target

Summary: Prevent codemods like D69233274 from deleting this otherwise unreferenced file.

Test Plan: testhard

Reviewed By: justintrudell

Differential Revision: D69472311

fbshipit-source-id: 34d74d6d7eb89bcb8110ad3a5529ee350483b65b

antlir/antlir2/features/install/tests/BUCK

@@ -233,6 +233,12 @@ image_python_test(
     supports_static_listing = False,
 )
 
+# This binary was manually built and copied to true.precompiled.(x86_64|aarch64)
+# for the tests below
+export_file(
+    name = "true.c",
+)
+
 export_file(
     name = "true.precompiled",
     src = arch_select(