fix(core): always call callback in DefaultStream._flush

[merceyz]

What's the problem this PR addresses?

DefaultStream._flush doesn't call the callback if the stream isn't empty which, because node 15 waits for it, causes the event loop to become empty and the process exits early during workspaces foreach calls

Note that the tests catch this when they run under node 15 but I can't enable it because of #2232

Fixes #2214
Fixes #2074

How did you fix it?

Always call the callback.

Checklist

• I have read the Contributing Guide.
• I have set the packages that need to be released for my changes to be effective.
• I will check that all automated PR checks pass before the PR gets reviewed.

[AlCalzone]

This may be a stupid question, but how do I get this version? I'm on yarn 2.4.2, freshly installed, freshly imported the workspace-tools, but yarn workspaces foreach still only runs in the root:

$ yarn workspaces list -v
➤ YN0000: .
➤ YN0000: packages/config
➤ YN0000: packages/core
➤ YN0000: packages/maintenance
➤ YN0000: packages/serial
➤ YN0000: packages/shared
➤ YN0000: packages/testing
➤ YN0000: packages/zwave-js
➤ YN0000: Done in 0s 3ms

$ yarn workspaces foreach exec echo HELLO
←[94m➤←[39m ←[90mYN0000←[39m: HELLO

[merceyz]

Update to the latest RC

yarn set version 3.0.0-rc.6 && yarn plugin import workspace-tools

[AlCalzone]

Thanks that seems to work!