arcanist: Update certs to fix letsencrypt

Due to the old root certificate used by letsencrypt expiring, arcanist
could not connect anymore to servers using letsencrypt (like
reviews.llvm.org).

Fix it by using the default nix certificates.

(cherry picked from commit 93eb7786c9365981854a98e508d5b1cae5ee2fcf)

pkgs/development/tools/misc/arcanist/default.nix

@@ -1,4 +1,5 @@
 { bison
+, cacert
 , fetchFromGitHub
 , flex
 , php
@@ -46,6 +47,7 @@ stdenv.mkDerivation {
     make install -C support/xhpast
     make cleanall -C support/xhpast
     cp -R . $out/libexec/arcanist
+    ln -sf ${cacert}/etc/ssl/certs/ca-bundle.crt $out/libexec/arcanist/resources/ssl/default.pem
 
     ${makeArcWrapper "arc"}
     ${makeArcWrapper "phage"}