CI: Fix GHA path filters

Just saying `requirements.txt` caught all `requirements.txt` files in
all subdirectories. This patch anchors them to the repository root
folder.

.github/workflows/timeseries.yml

@@ -6,13 +6,13 @@ on:
     paths:
     - '.github/workflows/timeseries.yml'
     - 'topic/timeseries/**'
-    - 'requirements.txt'
+    - '/requirements.txt'
   push:
     branches: [ main ]
     paths:
     - '.github/workflows/timeseries.yml'
     - 'topic/timeseries/**'
-    - 'requirements.txt'
+    - '/requirements.txt'
 
   # Allow job to be triggered manually.
   workflow_dispatch: