move sudo to outside execute of system_requirements_internal

[lz100]

If we just want to print out the command with sudo but does not want to execute the command,
current 'pak' does not prepend "sudo" unless you combine execute = TRUE, e.g

pak::pkg_system_requirements("openssl", sudo = TRUE, execute = FALSE)

##  [1] "apt-get install -y libssl-dev"

This pull request moves sudo outside execute, so when sudo = TRUE, execute = FALSE, the sudo will still be prepend to
output.