fix post install script about execution error in proxy environment

[pillsilly]

Some explanation about why it would work in case you're confused by Got's document

In Got's readme it gives two example about setting proxy.

• using "tunnel"

await got('https://sindresorhus.com', { agent: { https: tunnel.httpsOverHttp({ proxy: { host: 'localhost' } }) } });

• or hpagent

await got('https://sindresorhus.com', { agent: { https: new HttpsProxyAgent({ keepAlive: true, keepAliveMsecs: 1000, maxSockets: 256, maxFreeSockets: 256, scheduling: 'lifo', proxy: 'https://localhost:8080' }) } });

But meanwhile if you check the "agent" chapter
it says

agent
Type: object

An object representing http, https and http2 keys for **http.Agent, https.Agent** and http2wrapper.Agent **instance**.

So the current proxy agent "'https-proxy-agent'" used by geckodriver
is actually feasible for "Got" as it's already a "http.Agent implementation"