fix: make the runner script posix compliant (#8197)

**Motivation**

Fix an issue identified while running script on the Linux. It was not
present on the Mac.

**Description**

- The `/#!/usr/bin/env sh` descriptor on top of the file result
differently on Mac and other machines, enforcing the script to be
treated in a particular shell script.
- On mac it's set to `bash`, but was not compliant with some versions of
the Linux.
- Use the POSIX style output redirect so script is compliant on most OS.

**Steps to test or reproduce**

- Try to run it locally

lodestar

@@ -4,6 +4,6 @@
 #
 # ./lodestar.sh beacon --network mainnet
 
-SCRIPT_DIR="$( cd "$( dirname "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )"
+SCRIPT_DIR=$(cd "$(dirname -- "$0")" >/dev/null 2>&1 && pwd -P)
 
 exec node --trace-deprecation --max-old-space-size=8192 "$SCRIPT_DIR/packages/cli/bin/lodestar.js" "$@"