Avoid ICE if environment variable is not set
Avoid ICE if environment variable is not set

Rustdoc could trigger a code path that relied on the
$CFG_COMPILER_HOST_TRIPLE environment variable being
set, causing an ICE if it was not. This fixes that,
emitting an error instead of crashing.

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