Set explicit srcs in hand-written bild build

I'm not sure when `lib.fileset` was introduced, but it allows me to easily
create a source that can be unpacked by the `unpackPhase` which also maintains
the directory structure. This means I can pass a subset of the whole repo to the
builder, and changing an unrelated file (outside of that subset) won't cause a
rebuild of bild, which wasn't the case before.

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