Link checkpoint on bootstrapping

[zhangchiqing]

Close https://github.com/onflow/flow-go/issues/6167.

This PR reduce bootstrapping time by creating soft links of checkpoint files instead of copying them. Could save ~10 mins for bootstrapping.

Tested on mainnet test EN, and worked.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 22 lines in your changes missing coverage. Please review.

Project coverage is 41.66%. Comparing base (ab87266) to head (2a85477).

Files	Patch %	Lines
ledger/complete/wal/checkpointer.go	0.00%	17 Missing :warning:
cmd/execution_builder.go	0.00%	5 Missing :warning:

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6173   +/-   ##
=======================================
  Coverage   41.65%   41.66%           
=======================================
  Files        1975     1975           
  Lines      139640   139653   +13     
=======================================
+ Hits        58172    58180    +8     
- Misses      75430    75437    +7     
+ Partials     6038     6036    -2     

Flag	Coverage Δ
unittests	41.66% <0.00%> (+<0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.