Consolidate required CI gate (#115)

tests/compat/build.rs

@@ -19,7 +19,14 @@ fn main() {
         .join("../..")
         .join(".env");
     println!("cargo:rustc-check-cfg=cfg(live_grid_credentials)");
-    println!("cargo:rerun-if-changed={}", dotenv.display());
+    // A missing rerun-if-changed input is perpetually stale to Cargo. CI and
+    // normal offline builds do not have this ignored credential file, so only
+    // watch it once it exists. RUN_LIVE_TESTS remains an unconditional watched
+    // input and therefore makes the documented opt-in rebuild discover a newly
+    // created .env file.
+    if dotenv.is_file() {
+        println!("cargo:rerun-if-changed={}", dotenv.display());
+    }
     for key in KEYS {
         println!("cargo:rerun-if-env-changed={key}");
     }