RunCommandLogs: identify and access via uuid

Using a sha1 of the command combined with the build ID is not a
particularly good or unique identifier:

* A build could fail, be restarted, and then succeed -- assuming no
configuration changes, the sha1 hash of the command as well as the build
ID will be the same. This would lead to an overwritten log file.

* Allowing user input to influence filenames is not the best of ideas.

src/lib/Hydra/Helper/Nix.pm

@@ -19,7 +19,6 @@ our @EXPORT = qw(
     cancelBuilds
     captureStdoutStderr
     captureStdoutStderrWithStdin
-    constructRunCommandLogFilename
     constructRunCommandLogPath
     findLog
     gcRootFor
@@ -591,13 +590,6 @@ sub isLocalStore {
 }
 
 
-sub constructRunCommandLogFilename {
-    my ($sha, $build_id) = @_;
-    my $filename = "$sha-$build_id";
-    return $filename;
-}
-
-
 sub constructRunCommandLogPath {
     my ($filename) = @_;