Fix syntax of Powershell environment variables
Fix syntax of Powershell environment variables

Use `${env:USERPROFILE}` rather than `%USERPROFILE%` - the latter is valid in CMD, but not in Powershell.
rust-lang/rustup,nodakai/rustup.rs,nodakai/rustup.rs,nodakai/rustup.rs,rust-lang/rustup,inejge/rustup.rs,inejge/rustup.rs,inejge/rustup.rs,rust-lang/rustup,inejge/rustup.rs,inejge/rustup.rs,rust-lang/rustup,nodakai/rustup.rs,nodakai/rustup.rs,rust-lang/rustup