Update install.sh script to use longer template name

[ProbstenHias]

Using the install.sh script fails on alpine distributions as the installed mktemp tool on alpine expects at least 6 X when creating a temp folder with a template.

According to the tool installed on alpine

ecc23e77fdf2:/# mktemp --help
BusyBox v1.36.1 (2023-11-07 18:53:09 UTC) multi-call binary.

Usage: mktemp [-dt] [-p DIR] [TEMPLATE]

Create a temporary file with name based on TEMPLATE and print its name.
TEMPLATE must end with XXXXXX (e.g. [/dir/]nameXXXXXX).
Without TEMPLATE, -t tmp.XXXXXX is assumed.
...

For that reason the install script fails on alpine with the error

mktemp: Invalid argument

This PR fixes the issue by adding two more x 😀

Fixes #767