Proposed environment variable to skip running tests

[rain1]

Hello!

I would like to propose an environment variable that can be used to skip 
tests, e.g. export GUIX_SKIP_TESTS=1

This could be implemented by changing line 297 of 
guix/guix/build/gnu-build-system.scm:

(define* (check #:key target (make-flags '()) (tests? (not target))
                (test-target "check") (parallel-tests? #t)
                #:allow-other-keys)
  (if (and tests? (not (getenv "GUIX_SKIP_TESTS"))) ;;;;;; HERE
      (zero? (apply system* "make" test-target
                    `(,@(if parallel-tests?
                            `("-j" ,(number->string 
(parallel-job-count)))
                            '())
                      ,@make-flags)))
      (begin
        (format #t "test suite not run~%")
        #t)))

The reason I suggest this is because installing GuixSD with --fallback 
is sometimes necessary (I was getting a lot of 504's errors, so had to 
use fallback) but in that case the subversion and git test suites take 
many hours and perform excessive grinding on hard disk. Allowing users 
the choice to skip tests in a situation like this means we can get 
guixsd installed quickly.