fix: getCredential(): load credentials with getClient()

[jkwlui]

This change is needed for implementing v4 signed URL. The request that is to be signed needs client_email before calling GoogleAuth.sign(), so we need to change it so that getCredentials() properly loads the credentials.

[codecov]

Codecov Report

Merging #648 into master will increase coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #648      +/-   ##
==========================================
+ Coverage   88.25%   88.39%   +0.14%     
==========================================
  Files          18       18              
  Lines         783      784       +1     
  Branches       86       86              
==========================================
+ Hits          691      693       +2     
+ Misses         80       79       -1     
  Partials       12       12

Impacted Files	Coverage Δ
src/auth/googleauth.ts	91.45% <100%> (+0.54%)	⬆️

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[codecov]

Codecov Report

Merging #648 into master will increase coverage by 0.14%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #648      +/-   ##
==========================================
+ Coverage   88.25%   88.39%   +0.14%     
==========================================
  Files          18       18              
  Lines         783      784       +1     
  Branches       86       86              
==========================================
+ Hits          691      693       +2     
+ Misses         80       79       -1     
  Partials       12       12

Impacted Files	Coverage Δ
src/auth/googleauth.ts	91.45% <100%> (+0.54%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ce4f99c...4fdd9b7. Read the comment docs.