Set rustup profile to minimal

[lqd]

This PR fixes #1511 by setting the minimal profile in the toolchain file, and removing the explicit places where this was already done in CI scripts.

I don't think using rustup set profile minimal as you mention in the issue works when there's already a rustup override file: I believe the override kinda sets the profile back to its default value.

[lqd]

Ah I didn't notice there was an open PR to fix the same issue. But since it's using the rustup set profile minimal trick, we can verify my assumption that it wouldn't work.

In that PR, for example in the ubuntu-20.04 dist job, components that are not in the minimal profile are still being downloaded:

info: downloading component 'rust-docs'

Compared to using the rust-toolchain file, in this PR's same job which does look to be correctly set to "minimal".