Fix prove_installcheck when used with PGXS

Fix prove_installcheck when used with PGXS

Commit 153e215677 added the portlock directory.  This is created in
$ENV{top_builddir} if it is set.  Under PGXS, top_builddir points into
the installation directory, which is not necessarily writable and in
any case inappropriate to use by a test suite.  The cause of the
problem is that the prove_installcheck target in Makefile.global
exports top_builddir, which isn't useful (since no other Perl code
actually reads it) and breaks this use case.  The reason this code is
there is probably that is has been dragged around with various other
changes, in particular a0fc813266, but without a real purpose of its
own.  By just removing the exporting of top_builddir in
prove_installcheck, the portlock directory then ends up under
tmp_check in the build directory, which is more suitable.

Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://www.postgresql.org/message-id/78d1cfa6-0065-865d-584b-cde6d8c18aff@enterprisedb.com