[8.x] Improvement for Log::build()

[Air-Petr]

This PR is connected with "On-Demand Channels" (https://laravel.com/docs/8.x/logging#on-demand-channels).
"On-Demand Channels" have been introduced by #39273

Problem

Every call of a Log::build(...) uses the same log channel. Example (can be executed in tinker):

use Illuminate\Support\Facades\Log;

// ... some code

Log::build([
  'driver' => 'single',
  'path' => storage_path('foo/custom.log'),
])->info('foo');

// Now we have a "foo" record in a "foo" directory

// ... some code

Log::build([
  'driver' => 'single',
  'path' => storage_path('bar/custom.log'),
])->info('bar');

// New "bar" record, but still in a "foo" directory

Case

We have a queue running. Some jobs need certain channels (based on the names from a database). We've tried to use "On-Demand Channels". And now all information goes to one channel (the one is used by a first job appeared in a queue).

Possible solution

We can extend a "build" method by adding second optional parameter - the name of a channel.
For me it's hard to say is it good or bad from architectural perspective. But it certainly can be useful in some situations.
I've prepared a code change and a test method.
I've run tests by calling:

./vendor/bin/phpunit --filter=LogManagerTest

[driesvints]

There's breaking changes here so please send this to master.

[Air-Petr]

@driesvints
Does an optional parameter considered as breaking change? It won't hurt existing functionality.

[driesvints]

Changing a method signature is a breaking change unfortunately.

[dennisprudlo]

@Air-Petr classes that extend LogManager and overwrite the build method will break, because the signature does not match anymore. That's what is Breaking, unfortunately.