ci: Less volatile cache keys

[BYK]

Instead of using direct image hashes, only use hashes from migrations folders for each respective image for cache key generation. Should increase cache hit rate significantly as we don't have migrations much.

Also swaps the key order from sentry-snuba to snuba-senry assuming Snuba has less frequent migration additions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.45%. Comparing base (b439c67) to head (d60ad70).
Report is 1 commits behind head on master.

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3522   +/-   ##
=======================================
  Coverage   99.45%   99.45%           
=======================================
  Files           3        3           
  Lines         184      184           
=======================================
  Hits          183      183           
  Misses          1        1           

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