Make compiletest testing use the local sysroot
Make compiletest testing use the local sysroot

We already set `compiletest` to use the local sysroot in #68019, but
that missed the configuration for testing `compiletest` itself.

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