[CI][Docker][Cortex-M]Update ci_cortexm to Ubuntu 20.04 and update nrfjprog version to 10.18.1

[mehrdadh]

This PR updates nrfjprog to latest (10.18.1) to fix issues related to running microTVM tests inside docker. The main issue was that with older version commands like nrfjprog --com was not working even with docker --privileged.

As a result of this update, we need to update ubuntu version from 18.04 to 20.04.

cc @driazati @gromero

[tvm-bot]

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• cc @Mousius, @areusch, @driazati, @gigiblender, @leandron _{See #10317 for details}

_{Generated by tvm-bot}

[Mousius]

Is there any way we can validate this script before executing it? Or can we revert back to the previous way of adding the apt repos? Executing arbitrary scripts from the internet is generally a bad thing 😸

[mehrdadh]

From their website (https://apt.llvm.org/) I tried this:

deb http://apt.llvm.org/focal/ llvm-toolchain-focal main

But the issue was that this one is missing LLVM-13. We are not using llvm-13 in cortexm image but it was installing before.
cc @driazati

[mehrdadh]

I fixed this, thanks to @driazati pointers.

[Mousius]

Similarly here, there should be a hash we can check to ensure it's the file we expect?

[mehrdadh]

I got this file from NRF website and they don't provide any hash based on my understanding
https://www.nordicsemi.com/Products/Development-tools/nrf-command-line-tools/download
I'm not sure if there's other way to find a hash for this file

[mehrdadh]

Add SHA to verify the files

[mehrdadh]

@Mousius There was an issue with my branch name, which was not processing correctly to trigger the docker build. Here is the new PR
#13736