deployment: update get-envoy script and release hooks

[travisgroth]

Summary

• Refactor goreleaser hooks for split get/embed scripts
• Update get-envoy script to pull from our binary source
• Update get-envoy script to accept an optional target directory env var

Related issues

#1908

Checklist

• reference any related issues
• updated docs
• updated unit tests
• updated UPGRADING.md
• add appropriate tag (improvement / bug / etc)
• ready for review

[codeclimate]

Code Climate has analyzed commit 140f7ce and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #2111 (140f7ce) into master (3906b70) will decrease coverage by 0.0%.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           master   #2111     +/-   ##
========================================
- Coverage    60.3%   60.2%   -0.1%     
========================================
  Files         165     165             
  Lines       11152   11152             
========================================
- Hits         6725    6723      -2     
- Misses       3652    3654      +2     
  Partials      775     775             

Impacted Files	Coverage Δ
internal/controlplane/xdsmgr/xdsmgr.go	74.0% <0.0%> (-5.8%)	⬇️
pkg/storage/inmemory/backend.go	86.7% <0.0%> (-3.7%)	⬇️
pkg/storage/inmemory/stream.go	73.4% <0.0%> (+4.0%)	⬆️
pkg/grpc/databroker/syncer.go	96.2% <0.0%> (+6.2%)	⬆️