Make global transforms cache additive only

[lptr]

Currently global transforms are executed in a workspace that is directly inside the cache directory. If a transform fails for some environmental reason, during the next run the files in the workspace will be modified. This conflicts with keeping the cache directory additive (i.e. files can appear there but never removed or modified).

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cc: @gradle/build-cache

[lptr]

One way to do this is to run in a separate workspace and then move the files into the cache on success.

[stale]

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