refactor(db/build): drop source index, add event index

[wass3rw3rk]

source column was used by legacy deployment implementation so should be dropped. it also had high cardinality, so is likely relatively large on bigger installations.

event has become more of a bottleneck for some queries for very busy repos when querying builds by event. we recommend adding it.

the changes in this PR only affect new installations. to accommodate these in existing installations, drop the source index (concurrently on Postgres) and add the new one with CREATE INDEX CONCURRENTLY IF NOT EXISTS builds_event ON builds (event); (for Postgres).

probably still wise to create the index prior to upgrading unless you have the VELA_DATABASE_SKIP_CREATION or DATABASE_SKIP_CREATION flag set to true, since the default statements don't use CONCURRENTLY on Postgres.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56.64%. Comparing base (72319ae) to head (156f535).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1228   +/-   ##
=======================================
  Coverage   56.64%   56.64%           
=======================================
  Files         623      623           
  Lines       35280    35280           
=======================================
  Hits        19986    19986           
  Misses      14620    14620           
  Partials      674      674           

Files with missing lines	Coverage Δ
database/build/index.go	64.00% <100.00%> (ø)

[KellyMerrick]

lgtm