Prevent inputs from being garbage collected during evaluation

There is still a tiny window between the calls to nix-prefetch-* and
addTempRoot. This could be eliminated by adding a "-o" option to
nix-prefetch-*, or by not using those scripts at all (and use
addToStore directly).

src/lib/Hydra/Plugin/GitInput.pm

@@ -96,6 +96,8 @@ sub fetchInput {
         {uri => $uri, branch => $branch, revision => $revision},
         {rows => 1});
 
+    addTempRoot($cachedInput->storepath) if defined $cachedInput;
+
     if (defined $cachedInput && isValidPath($cachedInput->storepath)) {
         $storePath = $cachedInput->storepath;
         $sha256 = $cachedInput->sha256hash;
@@ -124,6 +126,9 @@ sub fetchInput {
         # FIXME: Don't use nix-prefetch-git.
         ($sha256, $storePath) = split ' ', grab(cmd => ["nix-prefetch-git", $clonePath, $revision], chomp => 1);
 
+        # FIXME: time window between nix-prefetch-git and addTempRoot.
+        addTempRoot($storePath);
+
         txn_do($self->{db}, sub {
             $self->{db}->resultset('CachedGitInputs')->update_or_create(
                 { uri => $uri