Bypass codegen for docs.rs build

[kesyog]

The codegen performed in the build script returns an error in the
docs.rs build environment, as write permissions seem to be locked down.

This PR skips the codegen steps in the build script if the DOCS_RS
environment variable is set.

I didn't find a simple way in my Googling to bypass codegen for all rustdoc
builds, but it doesn't seem to pose a real problem outside docs.rs anyway.

Testing

I didn't want to pull the 10GB needed to run the Docker images that
docs.rs uses. Instead, as a sanity check, I ran
cargo doc --all-features with and without the DOCS_RS environment
set and verified via strategically placed panic! macros that the
build script was bypassed when the variable was set 😬

References

• https://docs.rs/about/builds#detecting-docsrs
• docs.rs page: https://docs.rs/crate/google-cloud/0.2.1
• Failed build: https://docs.rs/crate/google-cloud/0.2.1/builds/366129

Fixes #51

[xmclark]

The build errors suggest there is an issue with the service account. The error reads like an "empty string" error. @Hirevo do you have suggestions? I'd love to see this crate working on docs.rs.

[kesyog]

Poking @Byron @mwilliammyers @Hirevo one more time

[Byron]

Thanks a lot for the fix and sorry for the wait!

Even though a few people would be able to merge this PR including myself that wouldn't help with getting it published to crates.io as this needs @Hirevo.

Would merging it help at all?

For context, I am not maintaining any of these crates but created the organization.

[kesyog]

Thanks for getting back. Merging it on its own wouldn't really do much as building the documentation locally already works. We'd need to deploy to crates.io afterward to get the benefit. I guess we do need @Hirevo.