CI: reduce parallelism for pytest.

In November 2022 we seemed to increase parallelism from 2 and 3 to 10!
That is a huge load for these CI boxes, and does explain some of our
flakes.

We only run in parallel because some tests sleep, but it's diminishing
returns (GH runners have 4 VCPUs, 16GB RAM).

This reduces it so:
- Normal runs are -n 4
- Valgrind runs are -n 2
- Sanitizer runs are -n 3

If I use my beefy build box (64BG RAM) but reduce it to 4 CPUs:

Time for pytest -n 5:
Time for pytest -n 4:
Time for pytest -n 3:

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

.github/workflows/ci.yaml

@@ -312,7 +312,7 @@ jobs:
       - name: Test
         env:
           SLOW_MACHINE: 1
-          PYTEST_PAR: 10
+          PYTEST_PAR: 4
           TEST_DEBUG: 1
           TEST_DB_PROVIDER: ${{ matrix.TEST_DB_PROVIDER }}
           TEST_NETWORK: ${{ matrix.TEST_NETWORK }}
@@ -430,7 +430,7 @@ jobs:
           COMPAT: 1
           CFG: ${{ matrix.CFG }}
           SLOW_MACHINE: 1
-          PYTEST_PAR: 10
+          PYTEST_PAR: 4
           TEST_DEBUG: 1
           TEST_DB_PROVIDER: ${{ matrix.TEST_DB_PROVIDER }}
           TEST_NETWORK: ${{ matrix.TEST_NETWORK }}
@@ -524,8 +524,9 @@ jobs:
         env:
           SLOW_MACHINE: 1
           TEST_DEBUG: 1
+          PYTEST_PAR: 2
         run: |
-          VALGRIND=1 uv run eatmydata pytest tests/ -n 3 ${PYTEST_OPTS}  ${{ matrix.PYTEST_OPTS }}
+          VALGRIND=1 uv run eatmydata pytest tests/ -n ${PYTEST_PAR} ${PYTEST_OPTS}  ${{ matrix.PYTEST_OPTS }}
       - name: Upload test results
         if: always()
         uses: actions/upload-artifact@v4
@@ -613,8 +614,10 @@ jobs:
         run: tar -xvjf cln-compile-clang-sanitizers.tar.bz2
 
       - name: Test
+        env:
+          PYTEST_PAR: 2
         run: |
-          uv run eatmydata pytest tests/ -n 2 ${PYTEST_OPTS}  ${{ matrix.PYTEST_OPTS }}
+          uv run eatmydata pytest tests/ -n ${PYTEST_PAR} ${PYTEST_OPTS}  ${{ matrix.PYTEST_OPTS }}
       - name: Upload test results
         if: always()
         uses: actions/upload-artifact@v4
@@ -737,7 +740,7 @@ jobs:
           COMPAT: 1
           CFG: ${{ matrix.CFG }}
           SLOW_MACHINE: 1
-          PYTEST_PAR: 10
+          PYTEST_PAR: 4
           TEST_DEBUG: 1
           TEST_DB_PROVIDER: ${{ matrix.TEST_DB_PROVIDER }}
           TEST_NETWORK: ${{ matrix.TEST_NETWORK }}