Allow for a personal token to be specified for self-updates (#5568)

* Allow for a personal token to be specified for self-updates

This change will allow for setting the $GITHUB_ACCESS_TOKEN environment variable with a Github personal access token, allowing e.g. for higher rate limits

* Refactor github request and add test

---------

Co-authored-by: Paulo Saraiva <pauloman@cern.ch>

internal/selfupdate/github_test.go

@@ -0,0 +1,37 @@
+package selfupdate
+
+import (
+	"context"
+	"net/http"
+	"testing"
+
+	rtest "github.com/restic/restic/internal/test"
+)
+
+func TestNewGitHubRequest(t *testing.T) {
+	ctx := context.Background()
+	url := "https://api.github.com/repos/restic/restic/releases/latest"
+	acceptHeader := "application/vnd.github.v3+json"
+
+	t.Run("With GITHUB_ACCESS_TOKEN", func(t *testing.T) {
+		expectedToken := "testtoken123"
+		t.Setenv("GITHUB_ACCESS_TOKEN", expectedToken)
+
+		req, err := newGitHubRequest(ctx, url, acceptHeader)
+		rtest.OK(t, err)
+
+		rtest.Assert(t, req.Method == http.MethodGet, "expected method %s, got %s", http.MethodGet, req.Method)
+		rtest.Assert(t, req.URL.String() == url, "expected URL %s, got %s", url, req.URL.String())
+		rtest.Assert(t, req.Header.Get("Accept") == acceptHeader, "expected Accept header %s, got %s", acceptHeader, req.Header.Get("Accept"))
+		rtest.Assert(t, req.Header.Get("Authorization") == "token "+expectedToken, "expected Authorization header 'token %s', got %s", expectedToken, req.Header.Get("Authorization"))
+	})
+
+	t.Run("Without GITHUB_ACCESS_TOKEN", func(t *testing.T) {
+		t.Setenv("GITHUB_ACCESS_TOKEN", "")
+
+		req, err := newGitHubRequest(ctx, url, acceptHeader)
+		rtest.OK(t, err)
+
+		rtest.Assert(t, req.Header.Get("Authorization") == "", "expected no Authorization header, got %s", req.Header.Get("Authorization"))
+	})
+}