golangci: update version to v1.50.1

This commit updates the tool `golangci-lint` to the latest version that
has fixed a bug in the linter `paralleltest`. Also `go mod tidy` is run
inside `tool`, and `nosnakecase` is disabled as it's deprecated.

tools/go.mod

@@ -4,7 +4,7 @@ go 1.16
 
 require (
 	github.com/btcsuite/btcd v0.23.3
-	github.com/golangci/golangci-lint v1.46.2
+	github.com/golangci/golangci-lint v1.50.1
 	github.com/ory/go-acc v0.2.6
 	github.com/rinchsan/gosimports v0.1.5
 )