Fix newsfile CI check (#993)

Turns out this check, introduced in #990, didn't actually work right.

scripts/check-newsfragments

@@ -13,7 +13,12 @@ matched=0
 
 # assume that anything in the changelogs directory which starts with a number
 # is intended as a newsfile.
-find changelogs -regex '.*/[0-9]+\.[^/]+$' | while read f; do
+#
+# (we use the < <(...) syntax rather than a regular pipe to avoid spawning a
+# subshell for the while loop, which would mean that changes to shell variables
+# are not propagated)
+#
+while read f; do
     basename=$(basename $f)
     dirname=$(dirname $f)
 
@@ -26,7 +31,7 @@ find changelogs -regex '.*/[0-9]+\.[^/]+$' | while read f; do
         # see if this newsfile corresponds to the right PR
         [[ -n "$pr" && "$basename" == "$pr".* ]] && matched=1
     fi
-done
+done < <(find changelogs -regex '.*/[0-9]+\.[^/]+$')
 
 if [[ -n "$pr" && "$matched" -eq 0 ]]; then
     echo -e "\e[31mERROR: Did not find a news fragment with the right number: expected changelogs/*/newsfragments/$pr.*\e[39m" >&2