feat(tools/depotfmt): add a treefmt wrapper with depot configuration

Right now the only included formatter is gofmt, but we will extend
this over time.

The version of treefmt is bumped to 0.3.0 (which supports custom
config files) until this lands in nixpkgs.

Change-Id: I1e1aafd05ec7427c616f90c90490c528ecb2615c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/4399
Tested-by: BuildkiteCI
Reviewed-by: grfn <grfn@gws.fyi>

tools/depotfmt.nix

@@ -0,0 +1,16 @@
+# Builds treefmt for depot, with a hardcoded configuration that
+# includes the right paths to formatters.
+{ pkgs, ... }:
+
+let
+  config = pkgs.writeText "depot-treefmt-config" ''
+    [formatter.go]
+    command = "${pkgs.go}/bin/gofmt"
+    options = [ "-w" ]
+    includes = ["*.go"]
+  '';
+in pkgs.writeShellScriptBin "depotfmt" ''
+  exec ${pkgs.treefmt}/bin/treefmt ''${@} \
+    --config-file ${config} \
+    --tree-root $(${pkgs.git}/bin/git rev-parse --show-toplevel)
+''