dev(ci): complete refactor

Nearly completely replace the original GitHub actions workflow.
Changes:

- Move from `.sh` files in `ci/steps` to steps in the workflow.
- Move from using docker images for environment to manual setup.
- Upgrade nfpm to v2.3.1

BREAKING CHANGE: official arm64 builds no longer support CentOS 7.
If you need to use CentOS 7 on arm64, build `code-server` locally.
For docs, see the yarn/npm section in `docs/install.md`.

package.json

@@ -68,6 +68,7 @@
     "parcel-bundler": "^1.12.4",
     "prettier": "^2.2.1",
     "prettier-plugin-sh": "^0.6.0",
+    "shellcheck": "^1.0.0",
     "stylelint": "^13.0.0",
     "stylelint-config-recommended": "^4.0.0",
     "ts-node": "^9.1.1",