[bugfix] Restore environment correctly in case of module load failures

[vkarak]

Fixes #1485.

[codecov-commenter]

Codecov Report

Merging #1501 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1501   +/-   ##
=======================================
  Coverage   91.57%   91.57%           
=======================================
  Files          83       83           
  Lines       12993    13002    +9     
=======================================
+ Hits        11898    11907    +9     
  Misses       1095     1095           

Impacted Files	Coverage Δ
reframe/core/runtime.py	94.33% <100.00%> (+0.07%)	⬆️
unittests/test_environments.py	73.12% <100.00%> (+1.22%)	⬆️

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