Allow passing environment variables through to the agave process

[SEJeff]

The metrics logging is a bit out of control and excessively logs.

# grep solana_metrics::metrics firedancer-since-09:30.log | grep -c 09:30:01
1328

With an agave instance, I can do RUST_LOG=info,solana_metrics=warn to suppress this, but I am unable to do the same for firedancer as the sandbox seems to be clearing them:

firedancer/src/util/sandbox/fd_sandbox.c

Lines 122 to 132 in 076b3cf

	void FD_FN_SENSITIVE
	fd_sandbox_private_explicit_clear_environment_variables( void ) {
	if( !environ ) return;
	for( char * const * env = environ; *env; env++ ) {
	ulong len = strlen( *env );
	explicit_bzero( *env, len );
	}
	if( clearenv() ) FD_LOG_ERR(( "clearenv failed" ));
	}

Can we add support for setting the env variables RUST_LOG and RUST_BACKTRACE to frankendancer for the agave bits until the full client is live?

I set that variable in the firedancer systemd unit and it didn't seem to be working. I confirmed this behaviour via:

# cat /proc/$(pgrep -f 'fdctl.*run-agave')/environ
#

This is an example of a single one of the log lines:

firedancer[20839]: INFO    2025-11-29 09:30:00.015801548 GMT+00  20838:7      firedancer:[${HOSTNAME_HERE}] :f10  fd1:[group]:0    metrics/src/metrics.rs(331)[solana_metrics::metrics]: datapoint: slot_stats_tracking_complete slot=128572897i last_index=63i num_repaired=0i num_recovered=0i min_turbine_fec_set_count=64i is_full=true is_rooted=true is_dead=false