[bugfix] Fix variable attribute injection

[jjotero]

Closes #2231

[codecov-commenter]

Codecov Report

Merging #2232 (8cbb6cf) into master (3f70d2b) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2232   +/-   ##
=======================================
  Coverage   85.54%   85.54%           
=======================================
  Files          55       55           
  Lines        9746     9746           
=======================================
  Hits         8337     8337           
  Misses       1409     1409           

Impacted Files	Coverage Δ
reframe/core/variables.py	96.10% <100.00%> (ø)

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