pacman-mirror: use the certificates of the 'ca-certificates' bundle

It appears to this developer as if the certificates shipped with the
mingw-w64-*-ca-certificates packages lag behind and ca-certificates is
much more up-to-date. At least twice were certificate issues with
Bintray solved by using the certificates bundle from the latter package,
despite being fully up-to-date on both ca-certificates packages.

So let's just hard-code the path to the bundle that is most likely to be
up-to-date.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

pacman-mirror.sh

@@ -19,6 +19,10 @@ die () {
 	exit 1
 }
 
+# MSys2's mingw-w64-$arch-ca-certificates seem to lag behind ca-certificates
+CURL_CA_BUNDLE=/usr/ssl/certs/ca-bundle.crt
+export CURL_CA_BUNDLE
+
 mode=
 case "$1" in
 fetch|add|push)