refactor(tvix): point SSL_CERT_FILE to /dev/null

reqwest wants to be able to read a file of trust roots when constructed,
but as it doesn't actually do any HTTPS connections inside the nix
build, an empty list of trust roots is totally sufficient.

Thankfully /dev/null provides such a file.

Change-Id: I9bd1619b2c9f8ff2a6640d2ac410d4de5b20c2ea
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11961
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: aspen <root@gws.fyi>

tvix/boot/tests/default.nix

@@ -53,7 +53,7 @@ let
         buildCommand = ''
           touch $out
           # Ensure we can construct http clients.
-          export SSL_CERT_FILE="${pkgs.cacert.out}/etc/ssl/certs/ca-bundle.crt"
+          export SSL_CERT_FILE=/dev/null
 
           ${preStart}