IMPALA-14606: (addendum) Install Python 3 for RHEL8

The first IMPALA-14606 commit miss to setup Python 3 in fresh RHEL8
machine. This was not caught before because I test using downstream
jenkins and it reuse RHEL8 machine that previously setup with Python 2.

This patch fix the issue by skipping pip install argparse that broke the
script and run setup_python3 instead for RHEL8 machine.

Testing:
- Run full bootstrap_system.sh and buildall.sh in fresh RHEL8 machine.

Change-Id: I6df0a534175404fe96d32eeb1e7bf0aa9ca204cd
Reviewed-on: http://gerrit.cloudera.org:8080/23772
Reviewed-by: Michael Smith <michael.smith@cloudera.com>
Reviewed-by: Laszlo Gaal <laszlo.gaal@cloudera.com>
Tested-by: Riza Suminto <riza.suminto@cloudera.com>

bin/bootstrap_system.sh

@@ -347,7 +347,7 @@ function setup_python2() {
 # IMPALA-14606: Stop building using Python 2 and always run with
 # IMPALA_USE_PYTHON3_TESTS=true.
 # redhat8 setup_python2
-redhat8 pip install --user argparse
+# redhat8 pip install --user argparse
 
 # Point Python to Python 3 for Redhat 9 and Ubuntu 22, or newer
 function setup_python3() {
@@ -378,6 +378,7 @@ function setup_python3() {
   fi
 }
 
+redhat8 setup_python3
 redhat9 setup_python3
 ubuntu22 setup_python3
 ubuntu24 setup_python3