[bugfix] Fix missing path to build stdout when collecting EasyBuild installed modules

[rsarm]

SUMMARY OF FAILURES
------------------------------------------------------------------------------
FAILURE INFO for BZip2Check 
  * Test Description: This demonstrates the EasyBuild build system.
  * System partition: daint:gpu
  * Environment: builtin
  * Stage directory: /users/sarafael/reframe-tmp/reframe/stage/daint/gpu/builtin/BZip2Check
  * Node list: None
  * Job type: batch job (id=None)
  * Dependencies (conceptual): []
  * Dependencies (actual): []
  * Maintainers: []
  * Failing phase: compile_wait
  * Rerun with '-n BZip2Check -p builtin --system daint:gpu -r'
  * Reason: file not found error: [Errno 2] No such file or directory: 'rfm_BZip2Check_build.out'

To reproduce it

import reframe as rfm
import reframe.utility.sanity as sn


class EasybuildMixin(rfm.RegressionTest):
    @rfm.run_before('run')
    def prepare_run(self):
        self.modules = self.build_system.generated_modules


@rfm.simple_test
class BZip2Check(EasybuildMixin):
    def __init__(self):
        self.descr = 'This demonstrates the EasyBuild build system.'
        self.valid_systems = ['dom:gpu', 'daint:gpu']
        self.valid_prog_environs = ['builtin']
        self.modules = ['EasyBuild-custom']
        self.build_system = 'EasyBuild'
        self.build_system.easyconfigs = ['bzip2-1.0.6.eb']
        self.build_system.options = ['-f']
        self.sanity_patterns = sn.assert_found(r'Version 1.0.6', self.stderr)
        self.executable = 'bzip2'
        self.executable_opts = ['--help']

[vkarak]

Wouldn't this be fixed if we simply switched to stagedir when calling post_build?

[rsarm]

@vkarak yes, I think so. Also I think we wouldn't need self._prefix_save.

[codecov-io]

Codecov Report

Merging #1891 (bfc78b3) into master (0208843) will decrease coverage by 0.03%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #1891      +/-   ##
==========================================
- Coverage   87.94%   87.90%   -0.04%     
==========================================
  Files          49       49              
  Lines        8453     8451       -2     
==========================================
- Hits         7434     7429       -5     
- Misses       1019     1022       +3     

Impacted Files	Coverage Δ
reframe/core/buildsystems.py	96.20% <0.00%> (-0.03%)	⬇️
reframe/core/pipeline.py	92.00% <100.00%> (-0.03%)	⬇️
reframe/utility/typecheck.py	94.54% <0.00%> (-0.91%)	⬇️
reframe/core/schedulers/slurm.py	52.43% <0.00%> (-0.29%)	⬇️
reframe/utility/osext.py	84.71% <0.00%> (ø)
reframe/frontend/loader.py	92.50% <0.00%> (ø)

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[vkarak]

lgtm. I've also updated the documentation of post_build().