grafana-agent: fix building of go-modules.drv from scratch

Unbreaks building the go-modules.drv from source (read: not substituted
from cache.nixos.org).

Regression from cc4dcc08be

pkgs/servers/monitoring/grafana-agent/default.nix

@@ -70,6 +70,12 @@ buildGoModule rec {
     popd
   '';
 
+  # do not pass preBuild to go-modules.drv, as it would otherwise fail to build.
+  # but even if it would work, it simply isn't needed in that scope.
+  overrideModAttrs = (_: {
+    preBuild = null;
+  });
+
   # uses go-systemd, which uses libsystemd headers
   # https://github.com/coreos/go-systemd/issues/351
   env.NIX_CFLAGS_COMPILE = toString (lib.optionals stdenv.isLinux [ "-I${lib.getDev systemd}/include" ]);