Set environment variable necessary to be a custom events source

[inlined]

The admin SDK does/will read the CloudEvent source field from an environment variable so that firebase can inject an experiment ID, function ID, or service ID rather than requiring customers to encode their deployment name in their codebase (not even possible for extensions).

[codecov-commenter]

Codecov Report

Base: 56.19% // Head: 56.17% // Decreases project coverage by -0.01% :warning:

Coverage data is based on head (834a50f) compared to base (74c1b19). Patch coverage: 18.75% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5078      +/-   ##
==========================================
- Coverage   56.19%   56.17%   -0.02%     
==========================================
  Files         308      308              
  Lines       20737    20752      +15     
  Branches     4199     4206       +7     
==========================================
+ Hits        11653    11658       +5     
- Misses       8071     8081      +10     
  Partials     1013     1013              

Impacted Files	Coverage Δ
src/emulator/functionsEmulatorShared.ts	42.30% <14.28%> (-1.13%)	:arrow_down:
src/deploy/functions/prepare.ts	29.64% <22.22%> (-0.20%)	:arrow_down:
src/emulator/auth/state.ts	85.43% <0.00%> (+0.56%)	:arrow_up:

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[inlined]

Yes it does. Where would I put that?