Fix .gitignore: stop tracking ignored files

backend/lib/openzeppelin-contracts/hardhat/env-contract.js

@@ -0,0 +1,10 @@
+extendEnvironment(env => {
+  const { contract } = env;
+
+  env.contract = function (name, body) {
+    // remove the default account from the accounts list used in tests, in order
+    // to protect tests against accidentally passing due to the contract
+    // deployer being used subsequently as function caller
+    contract(name, accounts => body(accounts.slice(1)));
+  };
+});