Tidy up the Robo file further

RoboFile.php

@@ -163,10 +163,11 @@ class RoboFile extends \Robo\Tasks {
         // create a temporary directory
         $dir = $t->tmpDir().\DIRECTORY_SEPARATOR;
         // create a Git worktree for the selected commit in the temp location
-        $result = $this->taskExec("git worktree add ".escapeshellarg($dir)." ".escapeshellarg($commit))->run();
+        $result = $this->taskExec("git worktree add ".escapeshellarg($dir)." ".escapeshellarg($commit))->dir(BASE)->run();
         if ($result->getExitCode() > 0) {
             return $result;
         }
+        try {
             // get useable version strings from Git
             $version = trim(`git -C "$dir" describe --tags`);
             $archVersion = preg_replace('/^([^-]+)-(\d+)-(\w+)$/', "$1.r$2.$3", $version);
@@ -220,9 +221,11 @@ class RoboFile extends \Robo\Tasks {
             $t->addTask($this->taskPack($tarball)->addDir("arsse", $dir));
             // execute the collection
             $result = $t->run();
+        } finally {
             // remove the Git worktree
             $this->taskFilesystemStack()->remove($dir)->run();
-        $this->taskExec("git worktree prune")->run();
+            $this->taskExec("git worktree prune")->dir(BASE)->run();
+        }
         return $result;
     }