Fixes the following warning:
```
Warning: the flag --darwin-use-unencrypted-nix-store-volume is no longer needed and will be removed in the future.
````
This version fixes GC bugs, which can be triggered under memory
pressure. Since GitHub runners are at times memory constraint,
it would be good to get this out.
A typical CI machine will have fast internet access, but may not have
all the inputs already in the store to rebuild trivial derivations
marked with `allowSubstitutes = false`. This leads to expensive cycles
of downloads and rebuilds for derivations like the top-level NixOS
system, which will be rebuilt from scratch on every CI run, despite
being cached.
Set `TMPDIR` if not already set to workaround potential disk space
issues while running builds in a nix shell. See NixOS/nix#395 for
more info.
fixes#197
The Nix installer produces a significant number of log messages, but
usually those messages are not really interesting. Group those messages
under a header, so that GitHub will keep them collapsed by default.
(Usually log messages are collapsed under the step title anyway, but if
this action is uses inside another composite action, there is no
separate step title, therefore the whole Nix installer output is
immediately visible when the log section for the outer composite action
is expanded. Adding the group header fixes this problem.)