copyStorePath(): Bail out early if the store path already exists

In rare cases (e.g. when using allowSubstitutes = false), it's
possible that we simultaneously have a DerivationGoal *and* a
SubstitutionGoal building the same path. So if a DerivationGoal
already built the path while the SubstitutionGoal was waiting for a
download slot, it saves us a superfluous download to exit early.

src/libstore/store-api.cc

@@ -981,6 +981,11 @@ void copyStorePath(
     RepairFlag repair,
     CheckSigsFlag checkSigs)
 {
+    /* Bail out early (before starting a download from srcStore) if
+       dstStore already has this path. */
+    if (!repair && dstStore.isValidPath(storePath))
+        return;
+
     auto srcUri = srcStore.getUri();
     auto dstUri = dstStore.getUri();
     auto storePathS = srcStore.printStorePath(storePath);