gen-index.sh: exclude .minisig sidecar files from the binaries list

Signed release binaries ship a bakery-x86_64.minisig alongside the
binary. The binaries-collection loop already filtered out .sha256/
.toml/.service/.css/.txt sidecars but not .minisig, so the signature
file itself got listed as an installable binary with no sha256 —
bakery then refused to install it (checksum mismatch) whenever it
tried to update a package with a signed binary.

scripts/gen-index.sh

@@ -56,6 +56,7 @@ build_package_json() {
         [[ "${bin_path}" == *.service ]] && continue
         [[ "${bin_path}" == *.css ]]     && continue
         [[ "${bin_path}" == *.txt ]]     && continue
+        [[ "${bin_path}" == *.minisig ]] && continue
         [[ -f "${bin_path}" ]] || continue
         local bin_name
         bin_name="$(basename "${bin_path}")"