install.sh: install --clone <git-url> [--dir] [--ref]

Clones your own config repository (git@host:owner/repo, https://...,
github:owner/repo) into ~/nix-config or --dir and installs it like
--flake <dir>. An existing clone of the same repository (any URL form) is
reused and, after asking, pulled with --ff-only; another repository, a
non-clone or a different branch is refused. A failed clone hints at SSH
URLs and gh auth login. Uses SteamOS' git, else Nix's.

Checked against local bare repositories through a logging git.

docs/development.md

@@ -48,7 +48,8 @@ modules/
   session.nix                  switch: outer bus/runtime dir, user services on switch
   cleanup.nix                  switch: `cleanup --orphans`; steam-frame-nix-cleanup on PATH
   cleanup/package.nix          build: install.sh as a command (cleanup, steamvr-debugger-arm, restart-check)
-  cleanup/check.nix            test: install.sh cleanup and restart-check on fake home/runtime dirs
+  cleanup/check.nix            test: install.sh cleanup and restart-check on fake home/runtime dirs,
+                               install --clone against local bare repos
   portal.nix                   Steam session portal config (session.portalFix)
   portal/check.nix             test: KDE FileChooser on by default, absent when disabled
   applications-menu.nix        applications.menu link for KDE apps (session.applicationsMenu)