* Update CI nix version
For `--option http2 false` to take effect requires Nix 2.3.2.
CHANGELOG_BEGIN
CHANGELOG_END
* Set option `http2 = false` dev-env nix config
This is less likely to overlook an instance than manually adding
`--option http2 false` to each Nix invocation.
Setting `--option htt2p false` also had no effect on the multi-user Nix
installation on the Linux CI machines due to
```
WARNING: option '--disk_cache' was expanded to from both option '--config linux' (source /nix/store/2xnfb2l39d2b4nxw5vwmqz5hjwhw0caw-daml-bazelrc) and option '--config linux' (source /nix/store/2xnfb2l39d2b4nxw5vwmqz5hjwhw0caw-daml-bazelrc)
```
Co-authored-by: Andreas Herrmann <andreash87@gmx.ch>
