RunCommand: use helper functions to ensure filenames and paths are the same

Otherwise, it's possible someone updates the format in one place but not
the others, leading to broken or incorrect functionality.

src/lib/Hydra/Helper/Nix.pm

@@ -19,6 +19,8 @@ our @EXPORT = qw(
     cancelBuilds
     captureStdoutStderr
     captureStdoutStderrWithStdin
+    constructRunCommandLogFilename
+    constructRunCommandLogPath
     findLog
     gcRootFor
     getBaseUrl
@@ -589,4 +591,20 @@ sub isLocalStore {
 }
 
 
+sub constructRunCommandLogFilename {
+    my ($sha, $build_id) = @_;
+    my $filename = "$sha-$build_id";
+    return $filename;
+}
+
+
+sub constructRunCommandLogPath {
+    my ($filename) = @_;
+
+    my $hydra_path = Hydra::Model::DB::getHydraPath;
+    my $bucket = substr($filename, 0, 2);
+
+    return "$hydra_path/runcommand-logs/$bucket/$filename";
+}
+
 1;