Build: adds the pkg/errors dependency that was missing from go.mod (#20143)

This dependency was added a while back and exists in the vendor
directory but was never properly added to go.mod.

go.mod

@@ -47,6 +47,7 @@ require (
 	github.com/mattn/go-sqlite3 v1.11.0
 	github.com/opentracing/opentracing-go v1.1.0
 	github.com/patrickmn/go-cache v2.1.0+incompatible
+	github.com/pkg/errors v0.8.1
 	github.com/prometheus/client_golang v0.9.2
 	github.com/prometheus/client_model v0.0.0-20190129233127-fd36f4220a90
 	github.com/prometheus/common v0.2.0

pkg/extensions/main.go

@@ -9,6 +9,7 @@ import (
 	"github.com/grafana/grafana/pkg/models"
 	"github.com/grafana/grafana/pkg/registry"
 	_ "github.com/jung-kurt/gofpdf"
+	_ "github.com/pkg/errors"
 	_ "github.com/robfig/cron"
 	_ "github.com/robfig/cron/v3"
 	_ "github.com/stretchr/testify/require"