Convert home-manager to flakes

Instead of calling the home manager directly in the user's list, we add
a command for the user to invoke "hms" and have their environment setup
in the way that Greg likes.

overlays/hms.nix

@@ -0,0 +1,20 @@
+{ pkgs, ... }:
+
+pkgs.writeShellScriptBin "hms" ''
+set -eo pipefail
+# Build different targets with GUI or not
+if [ -z "$DISPLAY" ]; then
+	target="nogui"
+else
+	target="gui"
+fi
+
+# Build and switch
+echo "Building $(uname -m)-$target"
+dest=$(mktemp -d)
+pushd "$dest" > /dev/null
+nix build --impure /etc/nixos#homeConfigurations.$(uname -m)-$target.activationPackage
+./result/activate
+popd > /dev/null
+rm -r "$dest"
+''