fix(flake.nix): build go-migrate with only postgres driver (#26643)

Follow-up to #26584.

After the nixpkgs 25.05 update, the default go-migrate package panics at
startup due to its Snowflake driver before Postgres commands can run.
Coder only uses the migrate CLI for migration creation and local Postgres
migrations, so override the build tags to include only the Postgres driver.

flake.nix

@@ -246,6 +246,14 @@
             xcbuild
           ]);
 
+        migrate = pkgs.go-migrate.overrideAttrs (_oldAttrs: {
+          # Coder only needs migrate for migration creation and local Postgres
+          # migrations. The nixpkgs default build includes every database
+          # driver and is broken by a bad Go and driver combination, so rebuild
+          # only with the driver we need.
+          tags = [ "postgres" ];
+        });
+
         # The minimal set of packages to build Coder.
         devShellPackages =
           with pkgs;
@@ -277,7 +285,7 @@
             gnutar
             unstablePkgs.go_1_26
             gofumpt
-            go-migrate
+            migrate
             (pinnedPkgs.golangci-lint)
             gopls
             gotestsum