chromium: Do not rely on import-from-derivation

This has been introduced by me in 690a845d and discovered by @vcunat in
his comment over at:

https://github.com/NixOS/nixpkgs/commit/690a845de93a49e85480ad5fae492fa14658ce85#commitcomment-14209868



It's really a bit ugly to have builds running during evaluation, but
back when I made that commit the reason was to avoid having to shell
quote the hell out of it (see the comment in mkPluginInfo for the
reason).

Now we propagate plugin flags and environment variables as a list of
arguments in a plain file that's appended verbatim to makeWrapper, so
it shouldn't do any builds anymore during instantiation.

I have tested this with both just WideVine and just Flash enabled as
well as both in combination and none of the plugins and the output seems
correct. However I didn't test to run Chromium with the new
implementation.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Reported-by: Vladimír Čunát <vcunat@gmail.com>