t/aggregate-results: fix paste(1) invocation

When running `make test`, when missing prereqs the following is emitted:

    make aggregate-results
    usage: paste [-s] [-d delimiters] file ...
    fixed   1
    success 30066
    failed  0
    broken  218
    total   31274

POSIX says that `paste` requires a file operand; stdin was clearly
intended by 49da404 (test-lib: show missing prereq summary,
2021-11-20). Use it.

Signed-off-by: D. Ben Knoble <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: benknoble

t/aggregate-results.sh

@@ -44,7 +44,7 @@ then
 		tr -s "," "\n" |
 		grep -v '^$' |
 		sort -u |
-		paste -s -d ' ')
+		paste -s -d ' ' -)
 	if test -n "$unique_missing_prereq"
 	then
 		printf "\nmissing prereq: $unique_missing_prereq\n\n"