Previously, the build directory for building serenity components was a
temporary directory in /tmp which was generated whenever a different
serenity component was built.
Instead of doing that, Hack Studio now simply uses the Build/ directory
inside the Serenity repository, similar to what is done in host builds.
This makes it so we don't re-build when switching back and forth between
different components.
It also makes it easier to inspect the build products.
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