curl --retry-all-errors not available on ubuntu focal, breaking gradlew-fdroid

Not sure if we should fix this (i.e. support anything older than bullseye and jammy).

But we could do a check and only use it if curl supports the option in the script.

if curl --help all | grep -q -- --retry-all-errors; then
  curl ... --retry-all-errors ...
else
  curl ...
fi

Cause: !1241 (merged).

It's supported by curl from bullseye and jammy. I checked that before I made the MR.