ci: Cache postgres volume after first migration

[BYK]

This patch caches all DB volumes based on the sentry and snuba images to avoid doing the same migrations over and over for every test run.

This shaved off a whole minute from "Install self-hosted" jobs (so ~20% speed increase).

Left side: cached re-run -- Right side: no-cache initial run

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.06%. Comparing base (282410a) to head (f41a7e6).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3488   +/-   ##
=======================================
  Coverage   98.06%   98.06%           
=======================================
  Files           3        3           
  Lines         207      207           
=======================================
  Hits          203      203           
  Misses          4        4           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.