flake/dev: pin flake-compat to PR fixing shallow repo

Pin flake-compat to edolstra/flake-compat#75
which fixes edolstra/flake-compat#74

If the flake is a shallow git repo, such as when fetched in CI,
flake-compat was failing to run in nix-instantiate.

Flake lock file updates:

• Updated input 'flake-compat':
    'https://api.flakehub.com/f/pinned/edolstra/flake-compat/1.1.0/01948eb7-9cba-704f-bbf3-3fa956735b52/source.tar.gz?narHash=sha256-NeCCThCEP3eCl2l/%2B27kNNK7QrwZB1IJCrXfrbv5oqU%3D' (2024-12-04)
  → 'github:edolstra/flake-compat/b70558fcb45e248abd2a0add41cfb6464c94eb03?narHash=sha256-JtdaTv%2B1VIiZpLsULCSQte6Qg3ew5axbdpOYjo%2B8D2U%3D' (2025-07-12)

Author: MattSturgeon

flake/dev/flake.lock

@@ -8,7 +8,9 @@
     dev-nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
 
     # flake-compat is used by the root `default.nix` to allow non-flake users to import nixvim
-    flake-compat.url = "https://flakehub.com/f/edolstra/flake-compat/1.tar.gz";
+    #
+    # The pinned PR resolves an issue with shallow clones, such as those used by CI.
+    flake-compat.url = "github:edolstra/flake-compat?ref=pull/75/merge";
 
     # keep-sorted start block=yes newline_separated=yes
     devshell = {