fuzz: Avoid influence on the global RNG from peerman m_rng

This should avoid the remaining non-determistic code coverage paths.

Without this patch, the tool would report a diff (only when running
without libFuzzer):

cargo run --manifest-path ./contrib/devtools/deterministic-fuzz-coverage/Cargo.toml -- $PWD/bld-cmake/ $PWD/../qa-assets/fuzz_corpora/ p2p_headers_presync 32

src/test/fuzz/p2p_headers_presync.cpp

@@ -32,6 +32,12 @@ public:
         PeerManager::Options peerman_opts;
         node::ApplyArgsManOptions(*m_node.args, peerman_opts);
         peerman_opts.max_headers_result = FUZZ_MAX_HEADERS_RESULTS;
+        // The peerman's rng is a global that is re-used, so it will be re-used
+        // and may cause non-determinism between runs. This may even influence
+        // the global RNG, because seeding may be done from the gloabl one. For
+        // now, avoid it influencing the global RNG, and initialize it with a
+        // constant instead.
+        peerman_opts.deterministic_rng = true;
         // No txs are relayed. Disable irrelevant and possibly
         // non-deterministic code paths.
         peerman_opts.ignore_incoming_txs = true;