redirect fetch_source progress to stderr so command substitution captures only the srcdir path, not the 'fetching source' message — fixes broken 'cd' in nix-shell build

@@ -58,8 +58,11 @@5858 # ── fetch source archive ──────────────────────────────────────────────────────5959 6060 fetch_source() {61+  # This function's stdout IS the source path (captured via command62+  # substitution), so every diagnostic must go to stderr — otherwise the63+  # caller's `cd "$srcdir"` receives the progress text too and fails.6164   srcdir="$(mktemp -d /tmp/nina-src.XXXXXX)"62-  say "fetching source from Kepr..."65+  say "fetching source from Kepr..." >&26366   fetch_file "$FLAKE_URL" "${srcdir}/nina.tar.gz" \6467     || die "could not download source archive — check your connection"6568   tar -xzf "${srcdir}/nina.tar.gz" -C "$srcdir" --strip-components=1 \