Use more robust mechanism for unsetting environmtne variables

Parsing the output of "env" can be fragile for certain complex
environment.
Using compgen should be safer.

easybuild/tools/run.py

@@ -217,8 +217,10 @@ def create_cmd_scripts(cmd_str, work_dir, env, tmpdir, out_file, err_file):
     with open(env_fp, 'w') as fid:
         # unset all environment variables in current environment first to start from a clean slate;
         # we need to be careful to filter out functions definitions, so first undefine those
-        fid.write("unset -f $(env | grep '%=' | cut -f1 -d'%' | sed 's/BASH_FUNC_//g')\n")
-        fid.write("unset $(env | cut -f1 -d=)\n")
+        fid.write("for var in $(compgen -e); do\n    unset \"$var\"\ndone\n")
+        # also unset any bash functions
+        fid.write("for func in $(compgen -A function); do\n    if [[ $func != _* ]]; then\n"
+                  "        unset -f \"$func\"\n    fi\ndone\n")
 
         # excludes bash functions (environment variables ending with %)
         fid.write('\n'.join(f'export {key}={shlex.quote(value)}' for key, value in sorted(env.items())