fix: use 'watchman' from nixpkgs, delete hacks, enable on aarch64-linux

Summary: Thanks to the heroic effort of several nixpkgs contributors, watchman
was finally updated to a (much) more recent build, and aarch64 support came
along with it! This allows us to support aarch64-linux, delete our own binary-
based package, and delete some special case hacks in Buck, as well. Overall
a win.

Test Plan: use 'watchman' on my laptop

.envrc

@@ -78,8 +78,6 @@ export BUCK2_TEST_DISABLE_LOG_UPLOAD=true
 if [ "$(uname)" = "Darwin" ]; then
   # XXX FIXME (aseipp): macOS support
   echo "direnv: watchman: not enabling, because we're on macOS"
-elif [ "$(uname -m)" = "aarch64" ]; then
-  echo "direnv: watchman: skipping, because watchman isn't supported on aarch64-linux (yet)"
 elif [ ! -f .use_watchman ] && [ -z "$CI_RUNNING" ]; then
   # XXX FIXME (aseipp): prompt about this file to the user
   echo "direnv: watchman: not enabled, because .use_watchman is missing, and we're not in a CI build"

buck/nix/buck2/default.nix

@@ -51,11 +51,6 @@ in rustPlatform.buildRustPackage rec {
   dontStrip = true; # XXX (aseipp): cargo will delete dwarf info but leave symbols for backtraces
 
   patches = [
-    # XXX FIXME (aseipp): Disable watchman support entirely and always short-
-    # circuit to 'notify' on aarch64; this lets us keep things compatible on
-    # both aarch64-linux and x86_64-linux
-    ./aarch64-linux-notify-hack.patch
-
     # XXX FIXME (aseipp): Use a version of 'prost' with a new API for boxing
     # large structs that buck2 needs
     ./update-prost-fork.patch

buck/nix/flake.nix

@@ -117,16 +117,12 @@
             inherit (pkgs)
               coreutils curl # downloading and fundamental scripts
               tagref sapling jq getopt jujutsu # utilities
+              watchman # fs integration
               ;
 
             # Finally, any globally useful tools we package here go next. This is primarily
             # buck, in our case...
             buck2 = pkgs.callPackage ./buck2 { };
-          }) // (pkgs.lib.optionalAttrs (system == flake-utils.lib.system.x86_64-linux) {
-            # watchman is only supported on x86_64-linux for now; in theory it
-            # should be possible to port to aarch64-linux through the github
-            # release...
-            watchman = pkgs.callPackage ./buck2/watchman.nix { };
           });
 
           # The default Nix shell. This is populated by direnv and used for the