Add min_const_gen to "all stable features" test invocation

[connec]

I was in the process of making a PR to support min_const_gen without std when I saw #1173 already existed! This PR is intended to unblock/accelerate that, though it makes sense as an orthogonal change. If it's preferable to fix it in that PR, feel free to close this one 🙂

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• cda0465 Add min_const_gen to "all stable features" test invocation

  min_const_gen is a stable feature, but was not added to this test
  invocation when it was introduced.

  Unblocks Fix no_std compatibility issues #1173.

[vks]

Unfortunately, this also runs this feature on Rust 1.36, which does not support const generics yet.

[connec]

Good point! I've pushed a new commit, splitting out the 'all features' test into two conditional steps, one for MSRV and one for non-MSRV.

I thought to deduplicate the 1.36.0 literal, but GitHub Actions only supports doing so using environment variables, e.g.

env:
    MSRV: 1.36.0
...
toolchain: ${{ env.MSRV }}
...
if: ${{ matrix.toolchain }} != ${{ env.MSRV }}

However I didn't like the idea of using environment variables for that, lest they interfere with something. Happy to apply it though if you don't think it's a concern.

[vks]

Thanks! Your change looks good, it's the same approach we are using for nightly.