Revert the fetchurl nix derivation. Keep the static binary at
/var/lib/tuwunel-admin/tuwunel-admin (copied from jul11, v0.0.0).
Service already runs this version on heather (nix derivation was pushed
but never deployed).
Replace manually-copied /var/lib/tuwunel-admin/tuwunel-admin binary with a
nix derivation: fetchurl of GitHub release v0.1.0 x86_64-musl static binary
(sha256 verified against checksums.txt). Binary now lives in nix-store,
survives VM recreation. Config stays at /var/lib/tuwunel-admin/config.toml
(outside git, root:root 0644).