mkcert is not amd64 only

Bad advice is detected in readme
https://github.com/FiloSottile/mkcert/blob/1c1dc4ed27ed5936046b6398d39cab4d657a2d8e/README.md?plain=1#L81

We should avoid hard-coded to architecture. Don't say this is just example. Even smart Canonical peoples copy-past [it](https://github.com/canonical/cloud-init/blob/78a748808c5206d2c9cefb4d05c338425a3d2dc4/tests/integration_tests/datasources/test_nocloud.py#L365).

I see two options to fix:
1) Make if-then script based on `dpkg --print-architecture` output or similar relative to OS
2) The best idea is to use well-known tools to download from github, for example, [binup](https://github.com/KonishchevDmitry/binup).
Examples how to use binup
```
sudo ./binup install --project mxmlnkn/rapidgzip --path '/usr/bin'
./binup -v install --project FiloSottile/mkcert
```
But mkcert should be packed in archive!

So regardless to architecture, I suggest to put mkcert release in tar.gz. In this case we can avoid `chmod +x /usr/local/bin/mkcert` and use `binup`. Github release in archive is de-facto standard, most of popular software use this way ([example1](https://github.com/mxmlnkn/rapidgzip/releases), [example2](https://github.com/rustdesk/rustdesk/releases)).