tools: dynamically determine parallelism on GitHub Actions macOS

Refs: #45340 (comment)
PR-URL: #45350
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>

.github/workflows/test-macos.yml

@@ -58,6 +58,6 @@ jobs:
       - name: tools/doc/node_modules workaround
         run: make tools/doc/node_modules
       - name: Build
-        run: make build-ci -j3 V=1 CONFIG_FLAGS="--error-on-warn"
+        run: make build-ci -j$(getconf _NPROCESSORS_ONLN) V=1 CONFIG_FLAGS="--error-on-warn"
       - name: Test
-        run: make run-ci -j3 V=1 TEST_CI_ARGS="-p actions --measure-flakiness 9"
+        run: make run-ci -j$(getconf _NPROCESSORS_ONLN) V=1 TEST_CI_ARGS="-p actions --measure-flakiness 9"