fix: Update base branch correctly in av pr create (#206)

Fixes a bug where running `av pr create` or `av stack submit` won't edit the base branch of a pull request that already exists.

I noticed this when I reparented a branch that I'd already created a PR for and noticed it didn't set the base branch correctly on GitHub.

internal/actions/pr.go

@@ -453,6 +453,7 @@ func ensurePR(
 			PullRequestID: opts.existingPR.ID,
 			Title:         gh.Ptr(githubv4.String(opts.title)),
 			Body:          gh.Ptr(githubv4.String(newBody)),
+			BaseRefName:   gh.Ptr(githubv4.String(opts.baseRefName)),
 		})
 		if err != nil {
 			return nil, false, errors.WithStack(err)