nixos/activation: Fix shellcheck test failure

lib/test.nix relies on `fileset.toSource` which creates a source dir
with the files of interest. `testers.shellcheck` passes all the files in
the source dir to shellcheck. The issue is related to relative path
sourcing, i.e. `source ./lib.sh` where shellcheck cannot make any
assumptions about the working directory.

Options were:
1) Disable this warning with a directive
   Prior disabling in the tree:
    - pkgs/tools/nix/info/info.sh
    - nixos/modules/testing/test-instrumentation.nix

2) Set source-path to SCRIPTDIR with a directive
   https://github.com/koalaman/shellcheck/wiki/Directive#source-path

   Even though we don't enable external script following for shellcheck
   with `-x` flag given every file in the source dir is passed, this
   directive seems to capture the intent to help shellcheck a bit
   better.

Went with option 2.