ledger-tool: modify-genesis command now writes elsewhere and produces a full genesis.tar.bz2

[mvines]

Just producing a genesis.bin is not enough, we need the full tar.bz2. Also overwriting the old genesis is poor form.

[codecov]

Codecov Report

Merging #11899 into master will increase coverage by 0.1%.
The diff coverage is 91.5%.

@@            Coverage Diff            @@
##           master   #11899     +/-   ##
=========================================
+ Coverage    82.0%    82.1%   +0.1%     
=========================================
  Files         330      331      +1     
  Lines       77588    78054    +466     
=========================================
+ Hits        63688    64151    +463     
- Misses      13900    13903      +3