d8: --expose-gc flag duplicated

[ivankra]

Running with --hostType=d8, evalScript keeps appending --expose-gc flag over and over again:

eshost/lib/agents/d8.js

Line 21 in 1614fb1

this.args.push('--expose-gc');

This should guarded like with node

eshost/lib/agents/node.js

Line 66 in 1614fb1

if (!this.args.includes('--expose-gc')) {