Support projectId for cloud functions

[michelepatrassi]

I was following https://firebase.google.com/docs/firestore/solutions/schedule-export in order to export my firestore database, and I noticed that my projectId was undefined (resulting in TypeError: Cannot read property 'charCodeAt' of undefined from google-gax).

Don't know exactly when GCP_PROJECT is populated, but with this fix GCLOUD_PROJECT will be picked up for the cloud functions 🏄‍♂️

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[michelepatrassi]

@googlebot I fixed it

[googlebot]

CLAs look good, thanks!

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[samtstern]

@michelepatrassi thanks! This change LGTM. I think GCP_PROJECT is outdated/unused but it's possible it's still set in some situations and this doesn't cause any harm.