Allow using a custom Nim version when building the beacon node

defaults/main.yml

@@ -92,3 +92,7 @@ beacon_node_slashing_db_kind: 'v2'
 
 # Netkey file contents, optional.
 #beacon_node_netkey: '{"key":"json"}'
+
+# Which version of Nim to use for the build
+# By default, it would use the one specified by the pinned nimbus-build-system
+beacon_node_nim_commit: ''

templates/build.sh.j2

@@ -20,6 +20,10 @@ function buildBinaries() {
     # Control number of jobs used to lower impact on running nodes
     export MAKEFLAGS="-j{{ beacon_node_build_jobs | int }}"
 
+    {% if beacon_node_nim_commit != "" %}
+    export NIM_COMMIT={{ beacon_node_nim_commit }}
+    {% endif %}
+
     make update OVERRIDE=1
     make nimbus_beacon_node \
         LOG_LEVEL="{{ beacon_node_build_log_level }}" \