ethereum: Rollup includes browser ethers.js for node

[armaniferrante]

When building the ethereum package for node with rollup, we're including the browser field. See https://github.com/oasislabs/oasis-client/blob/master/packages/ethereum/rollup/rollup.config.ts#L35. This causes:

$ jest test/e2e
  console.log ../ethereum/dist/index.umd.js:8723
    WARNING: Missing strong random number source; using weak randomBytes

  console.log ../ethereum/dist/index.umd.js:8723
    Warning: XMLHttpRequest is not defined

We should obviously change this.

[armaniferrante]

Labeled as P2 as it's not critical for release 3. Fast follow.