vm_setup: Remove exec() call from install_git

The 65b8c30d meant to use exec as a mean to "reload" the running
shell, however, exec simply replaces running bin image with a new
one, in this case, another shell. Since this call doesn't return
(unless it fails) this would suddenly leave the caller of the
vm_setup.sh inside the interactive shell.

Instead of re-executing the shell, simply export new PATH pointing
at where new git binary should reside.

Change-Id: I9ab5ba7f8f88602160c8be0f017477e0d1a01c08
Signed-off-by: Michal Berger <michalx.berger@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/2790


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Community-CI: Mellanox Build Bot
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Karol Latecki <karol.latecki@intel.com>