Switch to new Nix bindings, update Nix for that

Implements support for Nix's new Perl bindings[1]. The current state
basically does `openStore()`, but always uses `auto` and doesn't support
stores at other URIs.

Even though the stores are cached inside the Perl implementation, I
decided to instantiate those once in the Nix helper module. That way
store openings aren't cluttered across the entire codebase. Also, there
are two stores used later on - MACHINE_LOCAL_STORE for `auto`,
BINARY_CACHE_STORE for the one from `store_uri` in `hydra.conf` - and
using consistent names should make the intent clearer then.

This doesn't contain any behavioral changes, i.e. the build product
availability issue from #1352 isn't fixed. This patch only contains the
migration to the new API.

[1] https://github.com/NixOS/nix/pull/9863

src/script/hydra-update-gc-roots

@@ -5,7 +5,6 @@ use warnings;
 use File::Path;
 use File::stat;
 use File::Basename;
-use Nix::Store;
 use Hydra::Config;
 use Hydra::Schema;
 use Hydra::Helper::Nix;
@@ -47,7 +46,7 @@ sub keepBuild {
         $build->finished && ($build->buildstatus == 0 || $build->buildstatus == 6))
     {
         foreach my $path (split / /, $build->get_column('outpaths')) {
-            if (isValidPath($path)) {
+            if ($MACHINE_LOCAL_STORE->isValidPath($path)) {
                 addRoot $path;
             } else {
                 print STDERR "    warning: output ", $path, " has disappeared\n" if $build->finished;
@@ -55,7 +54,7 @@ sub keepBuild {
         }
     }
     if (!$build->finished || ($keepFailedDrvs && $build->buildstatus != 0)) {
-        if (isValidPath($build->drvpath)) {
+        if ($MACHINE_LOCAL_STORE->isValidPath($build->drvpath)) {
             addRoot $build->drvpath;
         } else {
             print STDERR "    warning: derivation ", $build->drvpath, " has disappeared\n";