fix: Make the prometheus exporter use the cli fallback parsing

Since Slurm changed the output of its json format between versions, the exporter breaks while trying to parse the new format. The `-slurm.cli-fallback` option allows the exporter to pass a custom output format through the `--format` option, which should be more resilient against breaking changes.

snap/snapcraft.yaml

@@ -77,7 +77,7 @@ apps:
     install-mode: disable
     after: [munged]
   slurm-prometheus-exporter:
-    command: bin/prometheus-slurm-exporter
+    command: bin/prometheus-slurm-exporter -slurm.cli-fallback
     daemon: simple
     install-mode: disable
     after: [munged]