Ensure the right dependencies are available on TaskCluster

Unfortunately this "hack" is required because the add-ons
pipeline does not enable consumers to customize the installed
packages.

package.json

@@ -3,6 +3,7 @@
   "version": "1.1.0",
   "type": "module",
   "scripts": {
+    "prebuild": "node scripts/setupTaskcluster.js",
     "build": "rollup -c && npm run build-addon && web-ext --config=web-ext-config.cjs build --overwrite-dest && mv web-ext-artifacts/*.zip web-ext-artifacts/rally_core.xpi",
     "build-addon": "npm run glean && rollup -c rollup.config.addon.js --config-enable-data-submission",
     "build-local-addon": "npm run glean && rollup -c rollup.config.addon.js --config-disable-remote-settings --config-studies-list-url=/public/locally-available-studies.json",

scripts/setupTaskcluster.js

@@ -0,0 +1,24 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+import * as exec from "child_process";
+import { promisify } from "util";
+
+// Define an async/await version of "exec".
+const execAsync = promisify(exec.exec);
+
+if (!('TASK_ID' in process.env)) {
+  // If this is not running on TaskCluster, exit
+  // without an error code.
+  process.exit(0);
+}
+
+console.log("Running in TaskCluster. Running set-up.");
+
+// The TaskCluster instance on the Mozilla add-ons pipeline
+// does not allow image customization, so python3 venv cannot
+// be installed. That package is required for Glean to build.
+// We make sure to have all the dependencies in place when on
+// TaskCluster by running the commands manually.
+execAsync("apt-get -y install python3-venv");