Chore: move build command to importable package (#38726)

* move build command to importable package & clean up

pkg/build/git.go

@@ -0,0 +1,17 @@
+package build
+
+func getGitBranch() string {
+	v, err := runError("git", "rev-parse", "--abbrev-ref", "HEAD")
+	if err != nil {
+		return "main"
+	}
+	return string(v)
+}
+
+func getGitSha() string {
+	v, err := runError("git", "rev-parse", "--short", "HEAD")
+	if err != nil {
+		return "unknown-dev"
+	}
+	return string(v)
+}