20846: Fix shell command in env var.

20846: Fix shell command in env var.

With Ruby 3, something uses the MAKE var without the expected
shell-eval, so the number-of-processors trick stopped working.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tom@curii.com>

    make --jobs \$\(grep -c processor /proc/cpuinfo\) DESTDIR\=
    sitearchdir\=./.gem.20231120-15-fa6sx6 sitelibdir\=./.gem.20231120-15-fa6sx6
    clean
    make: invalid option -- 'c'
    Usage: make [options] [target] ...
    Options:
      -b, -m                      Ignored for compatibility.
      [...]