fix: do not modify options object, use defaultScopes

[alexander-fenster]

Regenerated the library using
gapic-generator-typescript
v1.2.1.

[codecov]

Codecov Report

Merging #1360 (cc7d10c) into master (b5cf449) will decrease coverage by 0.01%.
The diff coverage is 98.30%.

@@            Coverage Diff             @@
##           master    #1360      +/-   ##
==========================================
- Coverage   98.54%   98.53%   -0.02%     
==========================================
  Files          32       32              
  Lines       19194    19375     +181     
  Branches     1377     1367      -10     
==========================================
+ Hits        18915    19091     +176     
- Misses        276      280       +4     
- Partials        3        4       +1     

Impacted Files	Coverage Δ
dev/src/v1beta1/firestore_client.ts	97.79% <98.11%> (+0.02%)	⬆️
dev/src/v1/firestore_admin_client.ts	98.81% <98.32%> (-0.03%)	⬇️
dev/src/v1/firestore_client.ts	98.09% <98.44%> (-0.09%)	⬇️

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