libstore: fix race condition when creating state directories

Running (parallel?) nix in nix can lead to multiple instances trying
to create the state directories and failing on the createSymlink step,
because the link already exists.

`replaceSymlink` is already idempotent, so let's use that.

See also:
- https://github.com/NixOS/nix/pull/13368
- https://github.com/NixOS/nix/issues/2706

Change-Id: I7fadd0ce3c1ffcebc9d281c00e5b49c12af3d50b

lix/libstore/local-store.cc

@@ -119,7 +119,7 @@ LocalStore::LocalStore(LocalStoreConfig config)
     Path gcRootsDir = config_.stateDir + "/gcroots";
     if (!pathExists(gcRootsDir)) {
         createDirs(gcRootsDir);
-        createSymlink(profilesDir, gcRootsDir + "/profiles");
+        replaceSymlink(profilesDir, gcRootsDir + "/profiles");
     }
 
     for (auto & perUserDir : {profilesDir + "/per-user", gcRootsDir + "/per-user"}) {