libstore: drop Store::connect

there's no reason a store should not open a connection during init if it
needs to open any connection to function. delaying connection setup like
this makes openStore less deterministic and graceful fallback impossible

this caused test failures where the old behaviour was required to ensure
test output stability. because of course something like that must happen

Change-Id: I946adddee026f1f4c74b4699d730b4d1ac9a5072

tests/functional2/cli/test_daemon.py

@@ -25,5 +25,5 @@ def test_stdio_forward_failure(nix: Nix, files: Path):
         .run()
         .expect(1)
     )
-    assert "stream ended unexpectedly" in result.stderr_plain
+    assert "cannot open connection to remote store" in result.stderr_plain
     assert "Lix crashed" not in result.stderr_plain