Fix pkill path in sreh test (#10171)

pkill is not in /bin/ folder on Ubuntu, so gpfdist can't be killed
in sreh test. That would make gpfdist regression test fail

src/test/regress/input/sreh.source

@@ -133,7 +133,7 @@ on SEGMENT 0
 FORMAT 'text' (delimiter '|');
 
 CREATE EXTERNAL WEB TABLE gpfdist_sreh_stop (x text)
-execute E'(/bin/pkill gpfdist || killall gpfdist) > /dev/null 2>&1; echo "stopping..."'
+execute E'(pkill gpfdist || killall gpfdist) > /dev/null 2>&1; echo "stopping..."'
 on SEGMENT 0
 FORMAT 'text' (delimiter '|');
 

src/test/regress/output/sreh.source

@@ -229,7 +229,7 @@ execute E'((@bindir@/gpfdist -p 8080 -d @abs_srcdir@/data  </dev/null >/dev/null
 on SEGMENT 0
 FORMAT 'text' (delimiter '|');
 CREATE EXTERNAL WEB TABLE gpfdist_sreh_stop (x text)
-execute E'(/bin/pkill gpfdist || killall gpfdist) > /dev/null 2>&1; echo "stopping..."'
+execute E'(pkill gpfdist || killall gpfdist) > /dev/null 2>&1; echo "stopping..."'
 on SEGMENT 0
 FORMAT 'text' (delimiter '|');
 select * from gpfdist_sreh_stop;