build: forward SENTRY_BACKEND/TRANSPORT Make variables

[jpnurmi]

The make build target is super convenient for quickly testing the sentry-native example in such environments as headless Docker containers and RPI over SSH. However, as soon as you need anything else besides the default backend or transport, one needs to fall back to configuring CMake by hand.

This proposal allows selecting a specific backend and/or transport while enjoying the convenience of 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 4 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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