Disallow skipping tx-sync tests in CI

Disallow skipping tx-sync tests in CI

Previously, we'd always skip tx-sync tests if the
`BITCOIND_EXE`/`ELECTRS_EXE` environment variables would be unset. While
this is especially fine for local testing, we still want to enforce
tests failing if somehow the `bitcoind`/`electrs` downloading or caching
in CI stops working. Here, we therefore add a `CI_ENV` variable that
indicates we're indeed running in CI, and only skip if it's unset.