cmd/gomobile: add a flag to manually specify a work dir

[aeddi]

Added a flag to manually specify a work dir to avoid having to rebuild everything from scratch when using gomobile through go module.

Particularly useful in the context of a large project that takes several tens of minutes to build, especially when only one or two packages need to be recompiled.

Fixes golang/go#37902

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Patch Set 2: Run-TryBot+1

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