Download one copy of Chrome per parallel worker

[barrucadu]

We've started seeing the error

ChildProcess::LaunchError:
  Text file busy - /root/.webdrivers/chromedriver

Which sounds a lot like
titusfortner/webdrivers#77, but that fix was
applied in 2019 and it solved the issues we had then. This is a new
occurrence, possibly related to a new version of Chrome (as one came
out recently).

Downloading one copy of chrome for each test runner isn't great, but
it's acceptable if it fixes the issue.

---

The failure is nondeterministic, but 10 builds of this branch in a row have passed, so I think it's a solution.

[kevindew]

Great stuff @barrucadu ⭐

Do we think webdrivers is trying to write a new chromedriver file at the start of the tests then? I had a memory that we'd disabled that action but may be mis-remembering.

[barrucadu]

I think it's running multiple instances of chrome in parallel which are fighting over a socket(?) file. I don't think the issue is to do with downloading as such, as there's no error about an empty or corrupted zip file, which the previous issue had.

Using a different copy of chrome for each worker has the benefit of making each one use a different socket file, at the cost of downloading chrome multiple times; but it fixes the problem.