CI: Run benchmarks if PR is labeled with "run/benchmark" (#2958)

Co-authored-by: Wei Ji <[email protected]>

.github/workflows/benchmarks.yml

@@ -12,8 +12,9 @@ on:
     paths:
       - 'pygmt/**/*.py'
       - '.github/workflows/benchmarks.yml'
-  # Uncomment the 'pull_request' line below to trigger the workflow in PR
-  # pull_request:
+  # Run in PRs but only if the PR has the 'run/benchmark' label
+  pull_request:
+    types: [ opened, reopened, labeled, synchronize ]
   # `workflow_dispatch` allows CodSpeed to trigger backtest
   # performance analysis in order to generate initial data.
   workflow_dispatch:
@@ -28,7 +29,7 @@ concurrency:
 jobs:
   benchmarks:
     runs-on: ubuntu-22.04
-    if: github.repository == 'GenericMappingTools/pygmt'
+    if: github.repository == 'GenericMappingTools/pygmt' && (github.event_name != 'pull_request' || contains(github.event.pull_request.labels.*.name, 'run/benchmark'))
     defaults:
       run:
         shell: bash -l {0}