pgsql: Fix new COPY test of PL/pgSQL with VPATH builds

Fix new COPY test of PL/pgSQL with VPATH builds

The buildfarm has turned red after 1858b10 because VPATH builds need to
use "@abs_srcdir@" and not "@abs_builddir@" for paths coming directly
from the source tree.  The input file of the new test got that right,
but not the output file.

Per complaints from several buildfarm animals, including desmoxytes and
culicidae.  I have also reproduced the error by myself.

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/943b447d303e6d0a7635d39ac552550b77b96cef

Modified Files
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src/pl/plpgsql/src/output/plpgsql_copy.source | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)