Check missing BEACON environment variable

[foolo]

if BEACON is not defined in .env, the scrips 3_gen_both_zkeys.sh 3_gen_chunk_zkey.sh will fail with

> node ../node_modules/.bin/snarkjs zkey beacon ../build/twitter/partial_zkeys/twitter_1.zkey ../build/twitter/twitter_nonchunked.zkey 10 '-n=Final Beacon phase2'
Invalid number of parameters
Usage: 
        snarkjs zkey beacon <circuit_old.zkey> <circuit_new.zkey> <beaconHash(Hex)> <numIterationsExp>

when the command is executed as

NODE_OPTIONS='--max-old-space-size=56000' node ../node_modules/.bin/snarkjs zkey beacon "$PARTIAL_ZKEYS"/"$CIRCUIT_NAME"_1.zkey "$BUILD_DIR"/"$CIRCUIT_NAME".zkey $BEACON 10 -n="Final Beacon phase2"

in 3_gen_both_zkeys.sh or 3_gen_chunk_zkey.sh