Don't hard-code the path to nasm to target/windows/nasm/nasm.

[briansmith]

We need to have a way to configure the path to nasm:

• Document which version of nasm is used in CI, where to get it, and its sha256 hash. See https://github.com/briansmith/ring-toolchain/tree/windows for how this is done for the version used in CI.
• Provide a configuration mechanism for indicating the path to nasm. Currently for perl we have fn get_perl_exe() -> PathBuf { get_command("PERL_EXECUTABLE", "perl") for this, which lets the user use $PERL_EXECUTABLE to provide the path to perl, or else we just expect it to be in $PATH. I guess it wouldn't be the end of the world to do similar for nasm. I think we should provide a cfg option for this so that people can put it in their configuration file instead of writing a script to set $PATH. Whatever we decide, we can change what we do for perl to match.

[davehorner]

I can pull this out into another PR.
https://github.com/briansmith/ring/pull/2416/files

I just tested if the tools were there and used the nasm from the env. is there something special that I shouldn't be using a choco install nasm?1

configuration is unneeded if you rely on the env. unless there's something special going on. i'll help complete this work.