CGI.pm was removed from Perl core in v5.22, breaking the gitweb.cgi
script and thus NixOS' services.lighttpd.gitweb. Nixpkgs switched to
Perl 5.22 with a85ba820a4 ("perl: Make 5.22 the default").
Fix it by bringing in the Perl CGI module (and dependency HTMLParser).
The closure size of "nix-build -A git" increase by 1 MiB, to 201 MiB.