[CLI] Removed DISPLAY & USER to leak to pure shell (#1149)

## Summary
TSIA

## How was it tested?
- compile
- `./devbox shell --pure`
- `echo $USER`
- `echo $DISPLAY`

Author: mohsenari

internal/impl/devbox.go

@@ -1193,10 +1193,9 @@ func (d *Devbox) convertEnvToMap(currentEnv []string) (map[string]string, error)
 			continue
 		}
 		// handling special cases to for pure shell
-		// Passing HOME USER and DISPLAY for pure shell to leak through
-		// otherwise devbox binary won't work - this matches nix
-		// We include PATH to find the nix installation. It is cleaned for pure mode below
-		if !d.pure || key == "HOME" || key == "USER" || key == "DISPLAY" || key == "PATH" {
+		// Passing HOME for pure shell to leak through otherwise devbox binary won't work
+		// We also include PATH to find the nix installation. It is cleaned for pure mode below
+		if !d.pure || key == "HOME" || key == "PATH" {
 			env[key] = val
 		}
 	}