COMP: Patch asv.conf.json branches to origin/main at harness-stage

ITKPerformanceBenchmarking's asv.conf.json (at the pinned SHA
41bf1b9cfbaa1b146e1b3e5d3ad3571b2203ce1e, introduced by #111) still
hard-codes branches=['origin/master'], but ITK's default branch has
been 'main' for some time.  Running `asv run` against origin/main
therefore fails with 'Unknown branch origin/master in configuration'
and aborts before any benchmark executes.

Rather than block this workflow on an upstream fix in
ITKPerformanceBenchmarking, patch asv.conf.json in place during the
harness-stage step so it references origin/main.  The upstream fix
should still land in ITKPerformanceBenchmarking's asv.conf.json; once
it does, this patch becomes a no-op and can be removed.

Author: hjmjohnson

.github/workflows/perf-benchmark.yml

@@ -118,6 +118,13 @@ jobs:
             echo "       InsightSoftwareConsortium/ITKPerformanceBenchmarking#111)." >&2
             exit 1
           fi
+          # ITKPerformanceBenchmarking's asv.conf.json still hard-codes
+          # ['origin/master']; ITK's default branch is 'main'.  Patch in
+          # place so `asv run` doesn't fail with "Unknown branch
+          # origin/master in configuration".  The upstream fix belongs
+          # in ITKPerformanceBenchmarking's asv.conf.json directly.
+          python -c "import json,sys,pathlib; p=pathlib.Path(sys.argv[1]); c=json.loads(p.read_text()); c['branches']=['origin/main']; p.write_text(json.dumps(c,indent=2)+'\n')" \
+            "${HARNESS}/asv.conf.json"
           ln -sfn "${GITHUB_WORKSPACE}/itk-head" "${HARNESS}/itk-repo"
           python -m pip install -e "${HARNESS}/python"
           echo "harness=${HARNESS}" >> "$GITHUB_OUTPUT"