rustbuild: Fix panic message when musl-root not set
rustbuild: Fix panic message when musl-root not set

The previous panic message delivered when a musl target was specified
but musl-root was not specified incorrectly instructed the user to add
the musl-root key to the "build" section of config.toml. The key
actually needs to be added to the "rust" section.

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