nats-server: add mainProgram for nats-server

The following warning was being output when using `nats-server`:

```
trace: warning: getExe: Package "nats-server-2.9.21" does not have the meta.mainProgram attribute. We'll assume that the main program has the same name for now, but this behavior is deprecated, because it leads to surprising errors when the assumption does not hold. If the package has a main program, please set `meta.mainProgram` in its definition to make this warning go away. Otherwise, if the package does not have a main program, or if you don't control its definition, use getExe' to specify the name to the program, such as lib.getExe' foo "bar".
```

1 files changed, 1 insertions, 0 deletions

diff --git a/pkgs/servers/nats-server/default.nix b/pkgs/servers/nats-server/default.nix
index aafbc2d1e5a..d60af6c137d 100644
--- a/pkgs/servers/nats-server/default.nix
+++ b/pkgs/servers/nats-server/default.nix
@@ -23,6 +23,7 @@ buildGoModule rec {
 
   meta = with lib; {
     description = "High-Performance server for NATS";
+    mainProgram = "nats-server";
     homepage = "https://nats.io/";
     changelog = "https://github.com/nats-io/nats-server/releases/tag/v${version}";
     license = licenses.asl20;