[ci] use alternate bash comparison

The pattern `echo str | grep -qE pattern` likes to fail on the z CI here for
an unknown reason. Use `grep -qE pattern <<< str` instead.

Signed-off-by: Christopher Jones <tophj@linux.vnet.ibm.com>

contrib/download-frozen-image-v2.sh

@@ -63,7 +63,7 @@ fetch_blob() {
 			-D-
 	)"
 	curlHeaders="$(echo "$curlHeaders" | tr -d '\r')"
-	if echo "$curlHeaders" | grep -qE "^HTTP/[0-9].[0-9] 3"; then
+	if grep -qE "^HTTP/[0-9].[0-9] 3" <<<"$curlHeaders"; then
 		rm -f "$targetFile"
 
 		local blobRedirect="$(echo "$curlHeaders" | awk -F ': ' 'tolower($1) == "location" { print $2; exit }')"