refactor: move runtimeScript to separate file

[milahu]

move runtimeScript to separate file

why?

• make the bash script easier to read
• avoid escaping

how?

• substitute
  • example use in nix-ccache

also avoid the "escape + echo" pattern

    echo "${runtimeScriptEscaped}" > $out/bin/nix-portable.zip

by using temporary files

just my 2 cents. nice tool!

[DavHau]

Sounds like a good idea. A PR implementing this change would be welcome.