Auto merge of #34690 - alexcrichton:clarify-vcvars, r=brson
Auto merge of #34690 - alexcrichton:clarify-vcvars, r=brson

Clarify rustbuild + msvc + vcvars in README

The invocation of vcvars is only needed for versions of Visual Studio that
rustbuild or cmake doesn't understand, but if older versions are installed then
there's no need to call vcvars.

Closes #34576

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