migrator: errors.As requires pointer as input (sourcegraph#49596)

I ran into a panic locally which pointed at this line:

  panic: errors.As: target must be a non-nil pointer, found pgconn.PgError
  ...snip
  github.com/sourcegraph/sourcegraph/internal/database/migration/runner.(*Runner).createIndexConcurrently.func3({0x63afbe0, 0xc002cda2d0})
  /home/keegan/src/github.com/sourcegraph/sourcegraph/internal/database/migration/runner/run.go:501 +0x79

Test Plan: I'm unsure how to get that code path executing again, so just
assuming this is correct by inspection.

internal/database/migration/runner/run.go

@@ -498,7 +498,7 @@ pollIndexStatusLoop:
 				if err == nil {
 					return err
 				}
-				if !errors.As(err, pgErr) || pgErr.Code != "42P07" {
+				if !errors.As(err, &pgErr) || pgErr.Code != "42P07" {
 					return err
 				}