[bugfix] Fixes find_modules() behaviour with flat module scheme when an environment is a substring of another one

[casparvl]

If you use find_modules with an environment_mapping (as typically done in a flat module naming scheme) where one environment is a substring of the other (e.g. foss and fosscuda), find_modules will consider any regex specified for fosscuda to also be valid for foss. This PR fixes that.

Fixes #1960

[jenkins-cscs]

Can I test this patch?

[teojgo]

ok to test

[codecov-commenter]

Codecov Report

Merging #1961 (2ad0ace) into master (c118fec) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #1961      +/-   ##
==========================================
- Coverage   87.91%   87.86%   -0.06%     
==========================================
  Files          50       50              
  Lines        8638     8668      +30     
==========================================
+ Hits         7594     7616      +22     
- Misses       1044     1052       +8     

Impacted Files	Coverage Δ
reframe/utility/__init__.py	92.82% <0.00%> (ø)
reframe/frontend/cli.py	75.59% <0.00%> (-0.93%)	⬇️
reframe/frontend/statistics.py	95.45% <0.00%> (ø)
reframe/frontend/runreport.py	92.98% <0.00%> (+0.42%)	⬆️

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[vkarak]

ok to test

[vkarak]

Thanks @casparvl for your fix! It was indeed a bug. I think it has been a leftover from an earlier implementation that assumed lists. That's why you see environs in the for loop.

[victorusu]

lgtm