chromium/plugins: Use @var@ for passing variables

There is already a pull request from @colemickens, who has just reversed
the variable references $flash and $flashVersion but the fix is kinda
fragile as he points out himself in #12713.

The reason the wrong substition was made is that both variables begin
with the same name and we do a simple replace instead of a more
complicated one using builtins.match.

So staying simple but to still not raising issues with other variables
that begin with the same name I'm now using @var@ instead, like we use
in substituteAll and other substituters (like the ones in CMake or
autotools) deal with it.

Note that I'm not using $var$ here to make sure it doesn't get confused
with real shell variables.

So with this fix in place, the wrapper now has the following flags:

  --ppapi-flash-path=/nix/store/.../lib/libpepflashplayer.so
  --ppapi-flash-version=20.0.0.294

Previously we had (#12710):

  --ppapi-flash-path=/nix/store/.../lib/libpepflashplayer.so
  --ppapi-flash-version=/nix/store/...-binary-plugins-flashVersion

Thanks to @colemickens for reporting and putting up a pull request.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>
Fixes: #12710
Fixes: #12713