build: forward SENTRY_BACKEND/TRANSPORT Make variables

[jpnurmi]

Makes it convenient to select a specific backend and/or transport for the make build:

$ make build SENTRY_BACKEND=native

or whichever way you prefer:

$ SENTRY_TRANSPORT=none make build

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.29%. Comparing base (5ef4c14) to head (1da843d).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2070      +/-   ##
==========================================
+ Coverage   74.26%   74.29%   +0.02%     
==========================================
  Files         103      103              
  Lines       26709    26709              
  Branches     4864     4864              
==========================================
+ Hits        19836    19844       +8     
+ Misses       5528     5525       -3     
+ Partials     1345     1340       -5     

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