init: add MALLOC_ARENA_MAX=1 to systemd

This adds the `MALLOC_ARENA_MAX=1` environment variable as suggested in
https://github.com/bitcoin/bitcoin/blob/master/doc/reduce-memory.md#linux-specific
to the systemd service file definition.

Without this env var, memory usage can grow significantly especially when
`rpcthreads` is increased above its default value.

Closes bitcoin#24542.

contrib/init/bitcoind.service

@@ -23,6 +23,9 @@ ExecStart=/usr/bin/bitcoind -daemonwait \
                             -conf=/etc/bitcoin/bitcoin.conf \
                             -datadir=/var/lib/bitcoind
 
+# Avoid excessive memory usage by bitcoind
+Environment="MALLOC_ARENA_MAX=1"
+
 # Make sure the config directory is readable by the service user
 PermissionsStartOnly=true
 ExecStartPre=/bin/chgrp bitcoin /etc/bitcoin