[bugfix] Fix injection of env variables for non default craype

[ekouts]

This is a small bugfix, because currently all tests are skipped because of errors like this with --non-default-craype:

WARNING: skipping test 'MKLResolveTest': type error: __init__() got an unexpected keyword argument 'variables' (rerun with '-v' for more information)
Traceback (most recent call last):
  File "/users/eirinik/repos/reframe/reframe/core/decorators.py", line 74, in instantiate_all
    leaf_tests.append(test(*args, **kwargs))
  File "/users/eirinik/repos/reframe/reframe/core/meta.py", line 439, in __call__
    obj.__init__(*args, **kwargs)
  File "/users/eirinik/repos/reframe/reframe/core/hooks.py", line 97, in _fn
    getattr(obj, h.__name__)()
  File "/users/eirinik/repos/reframe/reframe/core/pipeline.py", line 972, in __pre_init__
    self.__deferred_rfm_init.evaluate()
  File "/users/eirinik/repos/reframe/reframe/core/deferrable.py", line 73, in evaluate
    ret = self._fn(*fn_args, **fn_kwargs)
  File "/users/eirinik/repos/reframe/reframe/core/pipeline.py", line 1040, in __rfm_init__
    'LD_LIBRARY_PATH': '$CRAY_LD_LIBRARY_PATH:$LD_LIBRARY_PATH'
TypeError: __init__() got an unexpected keyword argument 'variables'

[teojgo]

lgtm

[codecov-commenter]

Codecov Report

Base: 86.34% // Head: 86.33% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (8e86a76) compared to base (3664427).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2682      +/-   ##
==========================================
- Coverage   86.34%   86.33%   -0.01%     
==========================================
  Files          60       60              
  Lines       11135    11135              
==========================================
- Hits         9614     9613       -1     
- Misses       1521     1522       +1     

Impacted Files	Coverage Δ
reframe/core/pipeline.py	93.41% <ø> (ø)
reframe/frontend/executors/policies.py	92.50% <0.00%> (-0.28%)	⬇️

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