current_flake_rev canonicalizes to /nix/store/<hash>-<pname>; the
hash right after /nix/store/ is what varies between builds, the
trailing -<pname> is constant. slice(-12) was taking the tail, so
two different builds would very likely render the same truncated
string. slice the hash prefix out instead, with a plain head-slice
fallback for a non-store-path rev (e.g. a bare local dir in dev).
damocles caught this in review on PR #2869.
abffa5234d