[bugfix] Fix remote auto-detection crash

[vkarak]

If you try to auto-detect a remote partition, ReFrame will crash as follows:

Traceback (most recent call last):
  File "./bin/reframe", line 22, in <module>
    cli.main()
  File "/users/karakasv/Devel/reframe/reframe/frontend/cli.py", line 617, in main
    autodetect.detect_topology()
  File "/users/karakasv/Devel/reframe/reframe/frontend/autodetect.py", line 229, in detect_topology
    with runtime.temp_environment(modules=temp_modules,
NameError: name 'temp_modules' is not defined

This PR fixes this and introduces a unit test that tests that.

[vkarak]

@jenkins-cscs retry daint

[codecov-commenter]

Codecov Report

Merging #2189 (5b86aa4) into master (4cc8c39) will decrease coverage by 0.05%.
The diff coverage is 27.27%.

@@            Coverage Diff             @@
##           master    #2189      +/-   ##
==========================================
- Coverage   86.31%   86.26%   -0.06%     
==========================================
  Files          53       53              
  Lines        9557     9565       +8     
==========================================
+ Hits         8249     8251       +2     
- Misses       1308     1314       +6     

Impacted Files	Coverage Δ
reframe/frontend/autodetect.py	60.40% <27.27%> (-2.01%)	⬇️

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[ekouts]

Looks fine. I just have a small question before merging.