Easy Install Script: Upgrade wkhtmltox to 0.12.6 for all architectures

Issue: Easy Install

**Do the checklist before filing an issue:**
- [x] Did you retain the logfile (path of logfile is shared while the script is run)? We definitely **need** the logfile to debug any easy install related issues.
- [x] Is this something you can debug and fix? Send a pull request! Bug fixes and documentation fixes are welcome

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**Additional context**

[wkhtmltox 0.12.6](https://github.com/wkhtmltopdf/wkhtmltopdf/releases) was released Jun 11, 2020.

https://github.com/frappe/bench/pull/1077 requires 0.12.6 because it is the first version that supports Arm64.

Continuation of frappe/bench#897.

**Possible Solution**

If @gavindsouza approves, I can make PR for this. Note: 0.12.6 does not have checksums.

Start with the easy install script and locate every wkhtmltox architecture reference. Compare the current references with the 0.12.6 release and review frappe/bench#1077 plus the continuation of frappe/bench#897. Done means the script uses 0.12.6 for all supported architectures, while accounting for its lack of checksums.

Written by the indexing model from the issue text.