kvutils: When creating the reference ledger dump, do it less eagerly. (#6525)

Our build is intermittently failing on CI on the target,
`//ledger/participant-state/kvutils:reference-ledger-dump`.

Creating the reference ledger dump exercises ledger-on-memory with the
Ledger API Test Tool as a side effect. The real goal is to run any kind
of kvutils ledger and generate a load of data. With that in mind, the
idea is to be somewhat stable, not fast.

This decreases the number of concurrent tests running at once to
generate the reference dump from the number of CPUs (presumably 8 on CI,
though I'm not sure) to 4.

It does not change the number of concurrent tests when running
`//ledger/ledger-on-memory/conformance-test`. This will still default to
the number of CPUs.

CHANGELOG_BEGIN
CHANGELOG_END

ledger/participant-state/kvutils/BUILD.bazel

@@ -163,6 +163,7 @@ client_server_build(
     testonly = True,  # only test targets can depend on this.
     client = "//ledger/ledger-api-test-tool",
     client_args = [
+        "--concurrent-test-runs=4",
         "--timeout-scale-factor=20",
         "localhost:6865",
     ],