fix: return impersonated token as lastReceivedToken

[bshaffer]

No description provided.

[mruell]

(related: #605)

Looks good to me, only were thinking about setting lastImpersonatedAccessToken to null first in fetchAuthToken, in case a later call does not have serviceAccountImpersonationUrl.

But I guess this case shouldn't be possible, so this should be fine

Thanks a lot and best!

[bshaffer]

in case a later call does not have serviceAccountImpersonationUrl.

The null-coalescing (??) operator will return false if the variable hasn't been initialized, so this case is covered!

Thanks for your review

[dazuma]

LGTM