go.mod: fix compattest/peercfg version

Since this was only accessed via a go.mod replace directive, it worked
fine for tests, etc. running in galaxycache's repo, but, since the
replace directive doesn't propagate outside the original repo, go
commands doing deep resolution complain because they can't resolve the
zero-valued version.

Note: Since the dependency on peercfg is only via tests, this didn't
actually affect building anything depending on Galaxycache, just
commands like that run by GoLand at startup:
```sh
go list -modfile=go.mod -m -json -mod=mod all
```

Author: dfinkel

go.mod

@@ -4,7 +4,7 @@ go 1.24.10
 
 require (
 	github.com/golang/protobuf v1.5.4
-	github.com/vimeo/galaxycache/compattest/peercfg v0.0.0-00010101000000-000000000000
+	github.com/vimeo/galaxycache/compattest/peercfg v0.0.0-20251208211800-872fc1d003d6
 	github.com/vimeo/go-clocks v1.3.0
 	go.opencensus.io v0.22.5
 	golang.org/x/sync v0.17.0

go.sum

@@ -22,6 +22,8 @@ github.com/google/uuid v1.6.0/go.mod h1:TIyPZe4MgqvfeYDBFedMoGGpEw/LqOeaOT+nhxU+
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/stretchr/objx v0.1.0/go.mod h1:HFkY916IF+rwdDfMAkV7OtwuqBVzrE8GR6GFx+wExME=
 github.com/stretchr/testify v1.4.0/go.mod h1:j7eGeouHqKxXV5pUuKE4zz7dFj8WfuZ+81PSLYec5m4=
+github.com/vimeo/galaxycache/compattest/peercfg v0.0.0-20251208211800-872fc1d003d6 h1:MkoIE/Uc/HFHVnER8EdRtt+UZa1Mp29QMm0Hkn77KXo=
+github.com/vimeo/galaxycache/compattest/peercfg v0.0.0-20251208211800-872fc1d003d6/go.mod h1:wjtRHtwdHRhP/HkJSaDDfBscxHX4g+h3GP5xU9IFOb8=
 github.com/vimeo/go-clocks v1.3.0 h1:6mSvNYB4Z8qwYIz2Xdp5/NU4vMf4cY4koerEFxyJJOw=
 github.com/vimeo/go-clocks v1.3.0/go.mod h1:GQ0ptP6CcJpmMeR7zi75pfy2ED69M/5tX4+aVPPPyvY=
 go.opencensus.io v0.22.5 h1:dntmOdLpSpHlVqbW5Eay97DelsZHe+55D+xC6i0dDS0=