fix: panic on export zero-height because variable-length addresses

[dongsam]

Closes: #1014

Description

Panic occurs when gaia export --for-zero-height

gaia/app/export.go

Line 160 in 56db57c

addr := sdk.ValAddress(iter.Key()[1:])

because of current prepForZeroHeightGenesis function using addr := sdk.ValAddress(iter.Key()[1:]) to casting variable-length addresses,
It should be addr := sdk.ValAddress(stakingtypes.AddressFromValidatorsKey(iter.Key())) to cover addresses of length > 20 bytes ADR-028

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• Wrote unit and integration tests
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---

For admin use:

• Added appropriate labels to PR (ex. WIP, R4R, docs, etc)
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[codecov]

Codecov Report

Merging #1015 (88a8016) into main (913c020) will not change coverage.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##             main    #1015   +/-   ##
=======================================
  Coverage   10.44%   10.44%           
=======================================
  Files          11       11           
  Lines        1082     1082           
=======================================
  Hits          113      113           
  Misses        964      964           
  Partials        5        5           

[okwme]

going to merge this as golangci-lint works locally