Make rust version a single workflow variable

[NickLarsenNZ]

It would be better to change the version in one place, instead across many steps.

Example (stripped for brevity and clarity):

env:
  RUST_TOOLCHAIN_VERSION: "1.75.0"

jobs:
  run_clippy:
    steps:
      - uses: dtolnay/rust-toolchain@master
        with:
          toolchain: ${{ env.RUST_TOOLCHAIN_VERSION }}
          components: clippy

operator-rs already does so:

• https://github.com/stackabletech/operator-rs/blob/407cfec12469dba32ac1795a9ce7e62077a0650b/.github/workflows/build.yml#L20
• https://github.com/stackabletech/operator-rs/blob/407cfec12469dba32ac1795a9ce7e62077a0650b/.github/workflows/build.yml#L86-L89

[sbernauer]

@NickLarsenNZ are you sure this belongs in "Development: Done"? :)

[NickLarsenNZ]

@sbernauer, yeah, there is a PR in/waiting for review so I moved this issue along which is covered by it.

I could move it back, but it seems silly to have two things for it. Or I could remove it from the board and leave the PR on the board.

What do you suggest?

[sbernauer]

No strong opinion, but I think I would suggest only putting the needed PRs on the board

[lfrancke]

This is a problem with GitHub where you can't "nest" things in a project.
I'd really love to track both things (an issue and the PRs) but as long as we don't have that we somehow need to....do something. So far we've been inconsistent and sometimes kept the issue and PR on the board or only one of the two....