`@var@' in the file `src', writing the result to $out, where `var'
is any environment variable starting with a lowercase character.
Example:
genericSubstituter {
src = ./file;
foo = "bla";
shell = bash + "/bin/sh";
};
will replace `@foo@' with `bla' and `@shell@' with
`/nix/store/...-bash-.../bin/sh'.
svn path=/nixpkgs/trunk/; revision=6928
