Here it comes...

Author: apple502j

.editorconfig

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+root = true
+[*]
+end_of_line = lf

action.yml

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+name: "Scratch Addons"
+author: "apple502j"
+description: "Packs Scratch Addons extension."
+inputs:
+  ENVIRONMENT:
+    description: 'Environment'
+    required: true
+runs:
+    using: "composite"
+    steps:
+        - run: bash $GITHUB_ACTION_PATH/scripts/pack.sh
+          shell: bash
+          name: "Pack extension"

package.json

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+{
+  "name": "packer-script",
+  "version": "1.0.0",
+  "description": "Packs Scratch Addons.",
+  "main": "index.js",
+  "scripts": {},
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/ScratchAddons/packer-script.git"
+  },
+  "author": "apple502j",
+  "license": "GPL-3.0-or-later",
+  "bugs": {
+    "url": "https://github.com/ScratchAddons/packer-script/issues"
+  },
+  "homepage": "https://github.com/ScratchAddons/packer-script#readme",
+  "private": true,
+  "type": "module"
+}

scripts/manifest.js

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+import fs from "fs/promises";
+import path from "path";
+
+const GITHUB_WORKSPACE = process.env.GITHUB_WORKSPACE;
+const {default: generate} = await import(path.join(GITHUB_WORKSPACE, ".github/gen-manifest.js"));
+
+const ENVIRONMENT = process.env.ENVIRONMENT;
+
+const MANIFEST_PATH = path.join(GITHUB_WORKSPACE, "manifest.json");
+
+await fs.copyFile(MANIFEST_PATH, path.join(GITHUB_WORKSPACE, ".manifest.json.bak"));
+
+const manifestFile = await fs.readFile(MANIFEST_PATH, "utf8");
+const manifest = generate(ENVIRONMENT, JSON.parse(manifestFile));
+
+await fs.writeFile(MANIFEST_PATH, JSON.stringify(manifest), "utf8");

scripts/pack.sh

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+#!/bin/bash
+cd $GITHUB_WORKSPACE
+
+export ENVIRONMENT=$INPUT_ENVIRONMENT
+
+echo $(printf "Packing build for %s" $ENVIRONMENT)
+
+node $GITHUB_ACTION_PATH/scripts/manifest.js
+mkdir $GITHUB_WORKSPACE/.dist 2>/dev/null
+
+VERSION_NAME=$(cat $GITHUB_WORKSPACE/manifest.json | jq ".version_name" | sed "s/[.-]/_/g;s/prerelease/pre/")
+FILENAME=$(printf "ScratchAddons-%s-%s.zip" $VERSION_NAME $ENVIRONMENT) 
+
+git archive --format=zip -o $GITHUB_WORKSPACE/.dist/$FILENAME HEAD
+rm $GITHUB_WORKSPACE/manifest.json
+mv $GITHUB_WORKSPACE/.manifest.json.bak $GITHUB_WORKSPACE/manifest.json
+
+echo $(printf "Packed as %s" $FILENAME)