ci: reduce nproc multipliers

The timeout factor increase in #263 reliably causes SIGTERM/143 exit code errors.

https://github.com/bitcoin-core/libmultiprocess/actions/runs/23559105412/job/68593864277?pr=240
https://github.com/bitcoin-core/libmultiprocess/actions/runs/23559105412/job/68593864283?pr=240
https://github.com/bitcoin-core/libmultiprocess/actions/runs/23567207693/job/68621529932
https://github.com/bitcoin-core/libmultiprocess/actions/runs/23567207693/job/68621529938
https://github.com/bitcoin-core/libmultiprocess/actions/runs/23583136495/job/68670044896?pr=256
https://github.com/bitcoin-core/libmultiprocess/actions/runs/23583136495/job/68670044932?pr=256
https://github.com/bitcoin-core/libmultiprocess/actions/runs/23587830246/job/68685257102?pr=249
https://github.com/bitcoin-core/libmultiprocess/actions/runs/23587830246/job/68685257218?pr=249

Reduce the nproc multipliers to try and prevent that.

Author: Sjors

.github/workflows/bitcoin-core-ci.yml

@@ -34,7 +34,7 @@ jobs:
           - name: 'ASan + UBSan'
             unit_test_runs: 15
             functional_test_runs: 300
-            nproc_multiplier: 10
+            nproc_multiplier: 2
             functional_timeout_factor: 40
             runner: ubuntu-24.04
             apt-llvm: true
@@ -62,7 +62,7 @@ jobs:
           - name: 'macOS'
             unit_test_runs: 50
             functional_test_runs: 200
-            nproc_multiplier: 5
+            nproc_multiplier: 2
             functional_timeout_factor: 40
             runner: macos-15
             brew-packages: ccache capnp boost libevent sqlite pkgconf ninja
@@ -171,7 +171,7 @@ jobs:
           - name: TSan
             unit_test_runs: 8
             functional_test_runs: 300
-            nproc_multiplier: 10
+            nproc_multiplier: 2
             functional_timeout_factor: 40
 
     env: