Fix git info within VM for worktrees (#51119)

If a worktree is used, say for 7.x, and packaging tests are run, the
build within the VM will fail due to the parent checkout not being
accessible. This is because the path of the worktree is for the host
systtem, not the VM. This commit makes the git info unknown, just as if
the .git directory did not exist.

buildSrc/src/main/java/org/elasticsearch/gradle/info/GlobalBuildInfoPlugin.java

@@ -260,6 +260,9 @@ public class GlobalBuildInfoPlugin implements Plugin<Project> {
             } else {
                 // this is a git worktree, follow the pointer to the repository
                 final Path workTree = Paths.get(readFirstLine(dotGit).substring("gitdir:".length()).trim());
+                if (Files.exists(workTree) == false) {
+                    return "unknown";
+                }
                 head = workTree.resolve("HEAD");
                 final Path commonDir = Paths.get(readFirstLine(workTree.resolve("commondir")));
                 if (commonDir.isAbsolute()) {