[Upstream] Fix DUB issue 2012: Nested dub invocation behaves randomly

[Geod24]

Link: dlang/dub#2012

To do:

• Make sure nested DUB invocations use the provided compiler by default. Add documentation to code.dlang.org (repository here: https://github.com/dlang/dub-registry) explaining the behavior.
• Make sure the dub does not pick up the wrong dub.selections.json;
• Do not propagate any DFLAGS unless passed explicitly ($DUB $DFLAGS);

[omerfirmak]

PR for Issue 1: dlang/dub#2047

[omerfirmak]

PR#2: dlang/dub#2048

[omerfirmak]

What is the problem with dub.selections.json exactly? When I test with projects, not single file operation, dub seems to use the local dub.selections.json of each project. Is there a specific case with the single file mode?

[Geod24]

Oh the original message had a typo: I meant dub.settings.json (dub picks up the parent dub.settings.json). But this last bug does not actually block us, so this issue is done, thanks for your work!