prometheus: gross hack to fix linker flags for versioning info

Apparently the handling of `buildFlagsArray` in `buildGo*` is blatantly
broken since it doesn't quote flags specified as list elements properly.

Because of that, the `-ldflags` are not interpreted properly and
`prometheus --version` doesn't output anything useful. By specifying
flags in both `buildFlags` and `buildFlagsArray` the issue gets fixed
since both variables are passed to `go install`.

(cherry picked from commit b33b086)

pkgs/servers/monitoring/prometheus/default.nix

@@ -44,10 +44,10 @@ in buildGoPackage rec {
     ln -s ${webui} web/ui/static/react
   '';
 
+  buildFlags = "-tags=builtinassets";
   buildFlagsArray = let
     t = "${goPackagePath}/vendor/github.com/prometheus/common/version";
   in [
-    "-tags=builtinassets"
     ''
       -ldflags=
          -X ${t}.Version=${version}