fix(CI): enforce node 11 usage to match dockerfile

[benjamincharity]

ISSUES CLOSED: #1581

[codecov]

Codecov Report

Merging #1605 into release will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           release    #1605   +/-   ##
========================================
  Coverage    98.81%   98.81%           
========================================
  Files          126      126           
  Lines         4797     4797           
  Branches       832      798   -34     
========================================
  Hits          4740     4740           
  Misses          57       57

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[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (release@b589b4e). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             release    #1605   +/-   ##
==========================================
  Coverage           ?   98.81%           
==========================================
  Files              ?      126           
  Lines              ?     4797           
  Branches           ?      798           
==========================================
  Hits               ?     4740           
  Misses             ?       57           
  Partials           ?        0

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update b589b4e...d244c86. Read the comment docs.