Remove usage of bzip2

[Leonidas-from-XIV]

We currently use the bzip2 binary to compress release archive and the -j flag to tar to unpack bzip2 archives. This is problematic for portability and also inconvenient for platforms that don't ship with bzip2 (like Windows and increasingly Linux distributions since bzip2 is losing popularity to newer compression standards like XZ or Zstd that are used for packages these days).

As such we should replace our usage of bzip2. Alternatives could be camlbz2 (to keep the exact same compression format at the cost of staying with an increasingly exotic choice) or camlzip which supports gzip and sacrifice some compression ratio while using a popular library and a format that won't go away for decades.