Fix new COPY test of PL/pgSQL with VPATH builds

The buildfarm has turned red after 1858b105 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.

src/pl/plpgsql/src/output/plpgsql_copy.source

@@ -32,7 +32,7 @@ CONTEXT:  PL/pgSQL function inline_code_block line 3 at EXECUTE
 -- COPY FROM
 DO LANGUAGE plpgsql $$
 BEGIN
-  COPY copy1 FROM '@abs_builddir@/data/copy1.data';
+  COPY copy1 FROM '@abs_srcdir@/data/copy1.data';
 END;
 $$;
 SELECT * FROM copy1 ORDER BY 1;
@@ -46,7 +46,7 @@ SELECT * FROM copy1 ORDER BY 1;
 TRUNCATE copy1;
 DO LANGUAGE plpgsql $$
 BEGIN
-  EXECUTE 'COPY copy1 FROM ''@abs_builddir@/data/copy1.data''';
+  EXECUTE 'COPY copy1 FROM ''@abs_srcdir@/data/copy1.data''';
 END;
 $$;
 SELECT * FROM copy1 ORDER BY 1;