Tell Devel::Cover to ignore files outside current build directory

This issue comes up in the coverage results of Test-PDL:

-pdl-commits:#pdl- [Test-PDL] [build/push]: ∀S ✓ a8ba9e2 | Zaki Mughal [sivoais] | Use shared GitHub Actions from PDLPorters (#6)
https://github.com/ebaudrez/Test-PDL/actions/runs/1557475038
[@sivoais] one odd thing with the coverage above: it thinks that a file from the PDL distribution is part of the coverage
[@sivoais] a file that is located in ~/.cpanm ??
[@sivoais] i saw this locally as well, but didn't bother tracking it down in Devel::Cover
[@sivoais] so even though Test-PDL's coverage is really 100%, it comes out at 76% on Coveralls

Coveralls link is: https://coveralls.io/jobs/91303010.

This is possibly just a matter of adding a -ignore_re option to the cover
command, but will need to be tested.

$ $TEST_RUNNER_PREFIX $MYPERL -S cover -test -relative_only -gcov_chdir -report Coveralls
   ----------------------------------- ------ ------ ------ ------ ------ ------
  File                                  stmt   bran   cond    sub   time  total
  ----------------------------------- ------ ------ ------ ------ ------ ------
  ...PDL-2.063/Basic/Core/Types.pm.PL   34.1   19.2   20.0   33.3   43.0   31.0
  blib/lib/Test/Deep/PDL.pm            100.0   75.0    n/a  100.0    5.3   97.7
  blib/lib/Test/PDL.pm                 100.0  100.0  100.0  100.0   51.6  100.0
  Total                                 73.3   69.4   81.8   58.2  100.0   70.3
  ----------------------------------- ------ ------ ------ ------ ------ ------