Installer Scripts: use bash instead of sh (#34039)

* Installer Scripts: use bash instead of sh

The agentless version of this script uses `bash`.
Using `bash` over `sh` should help the maitenance of the script.
It's being used for the agentless version and the node join script
`sh` is sometimes an alias to `bash`, and testing this script in those
systems might lead to assume that the script is working fine, even tho
when running in a real `sh` it will fail.

* use the same expression that is being used in agentless

api/types/installers/installer.sh.tmpl

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/usr/bin/env bash
 
 set -eu
 
@@ -44,7 +44,7 @@ on_azure() {
     sudo apt-get install -y ${PACKAGE_LIST}
   elif [ "$ID" = "amzn" ] || [ "$ID" = "rhel" ]; then
     if [ "$ID" = "rhel" ]; then
-      VERSION_ID=$(echo "$VERSION_ID" | sed 's/\..*//') # convert version numbers like '7.2' to only include the major version
+      VERSION_ID=${VERSION_ID//\.*/} # convert version numbers like '7.2' to only include the major version
     fi
     sudo yum-config-manager --add-repo \
       "$(rpm --eval "https://yum.releases.teleport.dev/$ID/$VERSION_ID/Teleport/%{_arch}/{{ .RepoChannel }}/teleport.repo")"