build: Writing tile clobbers prev_blob_free flag

[clarisma]

• Tiles are written concurrently, on the assumption that each thread works on a different set of blocks
• This assumption fails when a free blob at the end of a segment is reallocated, in which case the prev_blob_free flag of the next blob is cleared
• A thread writing tile data into this next blob potentially resets this flag because it read an earlier version of the blob header
• Solution: When writing a tile, never write the blob header; let BlobStore handle this exclusively