format code with gofumpt

gofumpt (https://github.com/mvdan/gofumpt) provides a supserset of `gofmt` / `go fmt`,
and addresses various formatting issues that linters may be checking for.

We can consider enabling the `gofumpt` linter to verify the formatting in CI, although
not every developer may have it installed, so for now this runs it once to get formatting
in shape.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

registry/proxy/proxymanifeststore_test.go

@@ -271,5 +271,4 @@ func TestProxyManifests(t *testing.T) {
 	if env.manifests.authChallenger.(*mockChallenger).count != 2 {
 		t.Fatalf("Expected 2 auth challenges, got %#v", env.manifests.authChallenger)
 	}
-
 }