nix: add a flake.nix with a devshell that delegates to shell.nix

Handling of the flake.lock file is a bit tricky because nix wants to
force-add it to git when it does any changes to it. You can get around
that with sth like:

```shell
$ git update-index --assume-unchanged misc/nix/flake.lock
```

Use the devshell like this:

```shell
$ nix develop ./misc/nix -c <your shell>
```

Where I use fish for <your shell>. You can also leave out the -c
argument if the default shell is fine for you.

Bonus content:

You can prevent garbage collection of the devshell dependencies but
installing a profile (which acts as a GC root):

```shell
$ nix develop --profile path/to/some-profile ./misc/nix -c fish
```

For example, you could keep live profiles in a hidden directory in your
home.

Author: aljoscha

.gitignore

@@ -28,3 +28,9 @@ junit_*.xml
 perf.data*
 **/*.rej
 **/*.orig
+
+# This is un-orthodox, but adding it to the repo would "tie" it to each users
+# local version of nixpkgs. This way, we can all use the flake and have a
+# flake.lock that works well with the rest of the package versions on our
+# system.
+**/flake.lock

misc/nix/flake.nix

@@ -0,0 +1,23 @@
+# Copyright Materialize, Inc. and contributors. All rights reserved.
+#
+# Use of this software is governed by the Business Source License
+# included in the LICENSE file.
+#
+# As of the Change Date specified in that file, in accordance with
+# the Business Source License, use of this software will be governed
+# by the Apache License, Version 2.0.
+
+{
+  description = "Devshell for materialize that delegates to the legacy shell.nix";
+
+  inputs.flake-utils.url = "github:numtide/flake-utils";
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let pkgs = nixpkgs.legacyPackages.${system}; in
+      rec {
+        # we pass `pkgs` directly to shell.nix
+        devShell = import ./shell.nix { inherit pkgs; };
+      }
+    );
+}