fix: source lib.sh in docker-buildx-push for tagging version

[jsjoeio]

I didn't realize the Docker workflow needed VERSION to publish a tag. This fixes that workflow.

See #5022 (comment) for more context

Todos

• manually dispatch Docker publish workflow to push 4.2.0 version

Fixes #5022

[github-actions]

✨ code-server docs for PR #5024 is ready! It will be updated on every commit.

• Host: https://coder.com/docs/code-server/9a21bda
• Last deploy status: success
• Commit: 2f7a361
• Workflow status: https://github.com/coder/code-server/actions/runs/2031058254

[codecov]

Codecov Report

Merging #5024 (2f7a361) into main (ba1ddbd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #5024   +/-   ##
=======================================
  Coverage   71.30%   71.30%           
=======================================
  Files          30       30           
  Lines        1683     1683           
  Branches      373      373           
=======================================
  Hits         1200     1200           
  Misses        413      413           
  Partials       70       70           

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