[feat] Automatically detect location of Spack

[giordano]

This removes the need to set the SPACK_ROOT environment variable. Fixes #2097.

I'm not sure how to test this though as it requires a working Spack to generate
the script.

[jenkins-cscs]

Can I test this patch?

[giordano]

I guess the obvious question is: why don't you do

cmds = ['. $(spack location --spack-root)/share/spack/setup-env.sh']

? The answer is because I'm not sure that'd work on my system: the problem I was seeing in #2097 is that SPACK_ROOT was set in the login node, but not on the compute one, despite the fact it is in my .bashrc. Likewise, spack would be in the PATH on the login node but not on the compute one. However I'll need to do some more investigation about whether using $(spack location --spack-root) would work. I think that not requiring to set the variable SPACK_ROOT would be good anyways though.

[codecov-commenter]

Codecov Report

Merging #2123 (d735799) into master (91d5797) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2123   +/-   ##
=======================================
  Coverage   85.97%   85.97%           
=======================================
  Files          53       53           
  Lines        9362     9362           
=======================================
  Hits         8049     8049           
  Misses       1313     1313           

Impacted Files	Coverage Δ
reframe/core/buildsystems.py	96.20% <100.00%> (ø)

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[giordano]

Ok, I finally figured out what was my trouble in #2097: the job scripts had

#!/bin/bash

as shebang, so my init file wasn't loaded. The solution was to use the RFM_USE_LOGIN_SHELL environment variable. With this, SPACK_ROOT is now set, and spack is in the path.

This pull request replaces $SPACK_ROOT with spack location --spack-root, since we require spack to be in the PATH anyways, but Spack doesn't require SPACK_ROOT to be set

[teojgo]

Ok to test

[vkarak]

Lgtm. I'm not sure about whether we need the quotes or not.