Update CI workflows: replace benchmark with AirspeedVelocity, fix downgrade (#490)

- Delete postbench.yml and runbenchandupload.yml, replace with AirspeedVelocity benchmark.yml
- Add julia_version parameter to downgrade-compat action

Co-authored-by: Stefan Krastanov <stefan@krastanov.org>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people

.github/workflows/benchmark.yml

@@ -0,0 +1,22 @@
+name: Benchmarks
+on:
+  pull_request_target:
+    branches: [master, main]
+permissions:
+  pull-requests: write
+
+concurrency:
+  # group by workflow and ref; the last slightly strange component ensures that for pull
+  # requests, we limit to 1 concurrent job, but for the master branch we don't
+  group: ${{ github.workflow }}-${{ github.ref }}-${{ (github.ref != 'refs/heads/master' && github.ref != 'refs/heads/main') || github.run_number }}
+  # Cancel intermediate builds, but only if it is a pull request build.
+  cancel-in-progress: ${{ startsWith(github.ref, 'refs/pull/') }}
+
+jobs:
+  benchmark:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: MilesCranmer/AirspeedVelocity.jl@action-v1
+        with:
+          julia-version: '1'
+          tune: 'false'

.github/workflows/downgrade.yml

@@ -24,6 +24,7 @@ jobs:
       - uses: julia-actions/julia-downgrade-compat@v2
         with:
           skip: Pkg,TOML,InteractiveUtils,Random,LinearAlgebra
+          julia_version: ${{ matrix.version }}
       - uses: julia-actions/cache@v2
       - uses: julia-actions/julia-buildpkg@v1
       - uses: julia-actions/julia-runtest@v1