docs: update CONTRIBUTING.md add upx as optional prerequesite (#4427)

A test failed as I didn't had upx installed:

```
--- FAIL: TestRun (0.58s)
    upx_test.go:119: 
                Error Trace:    /home/cipri/git/goreleaser/internal/pipe/upx/upx_test.go:119
                Error:          Received unexpected error:
                                upx not found in PATH
                Test:           TestRun
FAIL
```


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CONTRIBUTING.md

@@ -20,6 +20,7 @@ Other things you might need to run the tests:
 - [Podman](https://podman.io/)
 - [Snapcraft](https://snapcraft.io/)
 - [Syft](https://github.com/anchore/syft)
+- [upx](https://upx.github.io/)
 
 Clone `goreleaser` anywhere: