[DFSM] Load pcluster envars in compute node user data to resolve cfn …

[gmarciani]

Description of changes

Load pcluster envars in compute node user data to resolve cfn bootstrap scripts, such as cfn-init.
The lack of this change impacts all OS, but AL2, because AL2 comes with its own installation of cfn bootstrap scripts already loaded in the PATH.

This change has been missed in #6003 because we validated that PR with AL2 only.

Tests

• Without this fix cluster creation on Ubuntu2204 fails because compute node user data fails due to cfn-init not in the PATH.
• With this fix the above creation succeeds.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b6733ba) 90.21% compared to head (7ce6a46) 90.21%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #6023   +/-   ##
========================================
  Coverage    90.21%   90.21%           
========================================
  Files          180      180           
  Lines        15822    15822           
========================================
  Hits         14274    14274           
  Misses        1548     1548           

Flag	Coverage Δ
unittests	90.21% <ø> (ø)

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