Problematic hardcoded relative path to modulescmd executable

[micaeljtoliveira]

While running the regression tests locally on my laptop, one test fails because payu assumes that the modulescmd executable is located in $MODULESHOME/bin, which is not the case for all the linux distros (e.g., Debian and Ubuntu). Obviously not a problem when running on Gadi, but could be annoying for someone else trying to use payu in a different machine.

PS: The CI does not see the problem because no environment modules are installed in the Ubuntu image being used.