after_install: use . not source in bourne shell (#4639)

Another dumb mistake when using bourne shell:
```
/var/lib/dpkg/info/nimbus-beacon-node.postinst: 23: source: not found
```

Signed-off-by: Jakub Sokołowski <jakub@status.im>

scripts/package_src/nimbus_beacon_node/after_install

@@ -3,7 +3,7 @@ set -e
 
 DISTRO="UNKNOWN"
 if [ -r /etc/os-release ]; then
-    source /etc/os-release
+    . /etc/os-release
     DISTRO="${ID}"
 fi