Move cloud_sql_proxy installation before code copy

[sritchie]

User code changes most frequently, so let's move the cloudsql installation before this step. @ajslone , what do you think?

[codecov]

Codecov Report

Merging #87 into master will increase coverage by 0.08%.
The diff coverage is 18.18%.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
+ Coverage   54.23%   54.31%   +0.08%     
==========================================
  Files          31       31              
  Lines        3188     3183       -5     
==========================================
  Hits         1729     1729              
+ Misses       1459     1454       -5     

Impacted Files	Coverage Δ
caliban/docker/build.py	32.71% <18.18%> (-1.08%)	⬇️
caliban/platform/gke/util.py	99.25% <0.00%> (+0.37%)	⬆️
caliban/platform/cloud/types.py	90.50% <0.00%> (+0.63%)	⬆️
caliban/util/auth.py	76.19% <0.00%> (+9.52%)	⬆️

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[ajslone]

LGTM