justfile: Pre-lock flake before home-manager switch
Lix has a long-standing bug where the in-process flake locker, used when
home-manager (and nixos-rebuild) need to update the lock mid-switch,
refuses to fetch `path:.` for the root flake — "cannot fetch input
'path:.' because it uses a relative path". This bites whenever a
freshly-added input forces a lock update during the switch.
`nix flake lock` invoked directly canonicalises the root path
differently and works fine. Add a `_relock` recipe that runs it, and
make `install`, `install-home`, and `revert` depend on it so the
lock is reconciled before the switch ever touches it. `update` already
runs `nix flake update` so it's unaffected.