mercurial: add CA cert to test env for libgit

This seems to be required for the tests to pass on Hydra.

Otherwise, the test suit fails with errors like:

```
feature pygit2 failed:  OpenSSL error: failed to load certificates:
error:00000000:lib(0):func(0):reason(0)
```

pkgs/applications/version-management/mercurial/default.nix

@@ -11,6 +11,7 @@
 , which
 , sqlite
 , git
+, cacert
 , gnupg
 }:
 
@@ -70,6 +71,7 @@ let
       git
       gnupg
     ];
+    SSL_CERT_FILE = "${cacert}/etc/ssl/certs/ca-bundle.crt";  # needed for git
     checkPhase = ''
       cat << EOF > tests/blacklists/nix
       # tests enforcing "/usr/bin/env" shebangs, which are patched for nix