Installer: cleanup of the lazy Nix removal and install --clone

- The nix.mount drop-in is also removed when the uninstall fails or is
  interrupted (exit trap), and when daemon-reload failed after writing it.
- --dir with a trailing slash no longer puts the temporary clone inside
  the directory it replaces.
- The template's repository uses Nix's git when SteamOS has none, so the
  template marker exists there too (--clone needs it).
- A failed Nix uninstall during the reinstall of a broken Nix says so.
- clone_config: one branch per case; shorter comments, help and docs.
- Tests: an interrupted uninstall, --dir with a trailing slash; a
  redundant "used as it is" case dropped.

template/home.nix

@@ -10,9 +10,8 @@
   programs.home-manager.enable = true; # the `home-manager` command
 
   # git credentials for HTTPS (`install.sh install --clone` uses them):
-  # GitHub through the GitHub CLI (log in once: gh auth login, works with
-  # 2FA), other hosts from ~/.git-credentials. git itself is SteamOS's;
-  # Home Manager only writes its config. Remove to manage git yourself.
+  # github.com through the GitHub CLI (gh auth login), other hosts from
+  # ~/.git-credentials. git is SteamOS's; Home Manager writes its config.
   programs.git = {
     enable = true;
     package = null;