Auto merge of #8079 - Timmmm:patch-1, r=alexcrichton
Auto merge of #8079 - Timmmm:patch-1, r=alexcrichton

Add note about converting triple case in environment variables

This wasn't obvious to me, since `CARGO_TARGET_x86_64-unknown-linux-gnu_LINKER` is a perfectly valid environment variable name. It's especially important to document environment variable names well because if you get it wrong it is just silently ignored.

rust-lang/cargo,rust-lang/cargo,nipunn1313/cargo,tsurai/cargo,nipunn1313/cargo,rust-lang/cargo,rustyhorde/cargo,tsurai/cargo,nipunn1313/cargo,eminence/cargo,rust-lang/cargo,rustyhorde/cargo,tsurai/cargo,tsurai/cargo,eminence/cargo,rustyhorde/cargo,nipunn1313/cargo,tsurai/cargo,eminence/cargo,tshepang/cargo,eminence/cargo,rustyhorde/cargo,tshepang/cargo,tshepang/cargo,tshepang/cargo