extensions: simplifies the platform plug

[soumyaDghosh]

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This PR simplifies the desktop command chain approach, also adds support for separate Platform plug which was also used previously.

Better to merge this before #5092

[mr-cal]

@ScarlettGatelyMoore dropped the "make-parameters": ["PLATFORM_PLUG=... lines yesterday in d1c5ecb#diff-8a03e9a3b9e1e0cfa55b428426f2adc16394d075e8adf80c3f487022e02bb4f6L701.

Is PLATFORM_PLUG and this PR still needed or can it be dropped?

[soumyaDghosh]

@ScarlettGatelyMoore dropped the "make-parameters": ["PLATFORM_PLUG=... lines yesterday in d1c5ecb#diff-8a03e9a3b9e1e0cfa55b428426f2adc16394d075e8adf80c3f487022e02bb4f6L701.

Is PLATFORM_PLUG and this PR still needed or can it be dropped?

PLATFORM_PLUG that she was using i.e PLATFORM_PLUG=runtime snap name isn't used anymore, this PR reintroduces it in a different way. I did suggested the changes that were needed to make this PR work with #5092 , but it probably got things complicated on her side. So, I think better to wait, and once her PR is ready, I can rework on things. This PR basically removes the hardcoded gnome-platform path in the command-chain from here and make the path configurable.

EDIT: @jardon tried to do the same thing in this PR.

[mr-cal]

Waiting on #5092 to land first sounds like a good plan. Thanks!