rustbuild: Don't dist docs if disabled
rustbuild: Don't dist docs if disabled

This commit skips the `docs` dist step if the `--disable-docs` flag is passed,
fixing a compile error seen on nightly.

GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust