The Yarn GPG key (6A010C5166006599AA17F08146C2130DFD2497F5) can no
longer be fetched from keyservers, it returns "no user ID". So the
signature verification fails. So let's just download yarn via npm and
simplify this.
<!-- CURSOR_SUMMARY -->
---
> [!NOTE]
> **Low Risk**
> Build-only change confined to the Dockerfile; main risk is potential
reproducibility/supply-chain differences from switching Yarn
installation method.
>
> **Overview**
> Updates `templates/base/e2b.Dockerfile` to install Yarn via `npm
install -g yarn@1.22.19` instead of downloading and GPG-verifying the
Yarn tarball, removing the keyserver dependency and associated
verification steps.
>
> Also normalizes the Node version env var syntax from `ENV NODE_VERSION
20.9.0` to `ENV NODE_VERSION=20.9.0`.
>
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
243ce4b47da851e3faf64e2329016ef7a9681632. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->
Tomas Srnka
1852298a1a