CI: move grafana-build into pkg/build (#105640)

* move grafana-build into pkg/build

pkg/build/daggerbuild/git/github.go

@@ -0,0 +1,44 @@
+package git
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"log"
+	"os"
+	"os/exec"
+	"strings"
+)
+
+// LookupGitHubToken will try to find a GitHub access token that can then be used for various API calls but also cloning of private repositories.
+func LookupGitHubToken(ctx context.Context) (string, error) {
+	log.Print("Looking for a GitHub token")
+
+	// First try: Check if it's in the environment. This can override everything!
+	token := os.Getenv("GITHUB_TOKEN")
+	if token != "" {
+		log.Print("Using GitHub token provided via environment variable")
+		return token, nil
+	}
+
+	// Next, check if the user has gh installed and *it* has a token set:
+	var data bytes.Buffer
+	var errData bytes.Buffer
+	ghPath, err := exec.LookPath("gh")
+	if err != nil {
+		return "", fmt.Errorf("GitHub CLI not installed (expected a --github-token flag, a GITHUB_TOKEN environment variable, or a configured GitHub CLI)")
+	}
+
+	//nolint:gosec
+	cmd := exec.CommandContext(ctx, ghPath, "auth", "token")
+	cmd.Stdout = &data
+	cmd.Stderr = &errData
+
+	if err := cmd.Run(); err != nil {
+		log.Printf("Querying gh for an access token failed: %s", errData.String())
+		return "", fmt.Errorf("lookup in gh failed: %w", err)
+	}
+
+	log.Print("Using GitHub token provided via gh")
+	return strings.TrimSpace(data.String()), nil
+}