IPFS_PATH is no longer used after 1.x

[hacdias]

While finally trying to upgrade IPFS Desktop to CTL 3.x, I noticed every test where we were setting the IPFS_PATH env var were failing and then looked at the code.

Before 1.0, ipfsd-ctl was using the parent process IPFS_PATH variable if it was set:

this._env = Object.assign({}, process.env, this.opts.env)
// ...
get env () {
    return this.path ? Object.assign({}, this._env, { IPFS_PATH: this.path }) : this._env
}

Right now, the only place where that variable is mentioned doesn't seem to use the parent's process:

this.env = merge({ IPFS_PATH: this.path }, this.opts.env)

/cc @hugomrdias