Small CI improvements (#13060)

# Objective

- Some CI jobs specifically use `macos-14`, as compared to the default
`macos-latest`.
- `macos-latest` is equivalent to `macos-12`, but may be updated in the
future.
- The CI job that tests on the minimum supported Rust version (MSRV)
uses environmental variables to save the toolchain version.
  - This specific usage is what step outputs were designed for.
- Both do the same thing, but step outputs can be checked by the [Github
Actions VSCode
Extension](https://marketplace.visualstudio.com/items?itemName=GitHub.vscode-github-actions).
- Some workflows have a `NIGHTLY_TOOLCHAIN` variable that let us pin the
nightly version, in case a new release breaks CI.

## Solution

- Document why certain actions required `macos-14`.
- Switch MSRV step to use step outputs.
- Add a small comment documenting the purpose of the `NIGHTLY_TOOLCHAIN`
environmental variable.

.github/workflows/ci.yml

@@ -9,6 +9,7 @@ on:
 
 env:
   CARGO_TERM_COLOR: always
+  # If nightly is breaking CI, modify this variable to target a specific nightly version.
   NIGHTLY_TOOLCHAIN: nightly
 
 concurrency:
@@ -70,6 +71,7 @@ jobs:
         run: cargo run -p ci -- lints
 
   miri:
+    # Explicity use MacOS 14 to take advantage of M1 chip.
     runs-on: macos-14
     timeout-minutes: 60
     steps:
@@ -228,6 +230,7 @@ jobs:
 
 
   run-examples-macos-metal:
+    # Explicity use MacOS 14 to take advantage of M1 chip.
     runs-on: macos-14
     timeout-minutes: 30
     steps:
@@ -388,12 +391,13 @@ jobs:
             target/
           key: ${{ runner.os }}-cargo-msrv-${{ hashFiles('**/Cargo.toml') }}
       - name: get MSRV
+        id: msrv
         run: |
           msrv=`cargo metadata --no-deps --format-version 1 | jq --raw-output '.packages[] | select(.name=="bevy") | .rust_version'`
-          echo "MSRV=$msrv" >> $GITHUB_ENV
+          echo "msrv=$msrv" >> $GITHUB_OUTPUT
       - uses: dtolnay/rust-toolchain@master
         with:
-          toolchain: ${{ env.MSRV }}
+          toolchain: ${{ steps.msrv.outputs.msrv }}
       - name: Install Linux dependencies
         uses: ./.github/actions/install-linux-deps
       - name: Run cargo check

.github/workflows/docs.yml

@@ -10,6 +10,7 @@ on:
 env:
   CARGO_TERM_COLOR: always
   RUSTDOCFLAGS: --html-in-header header.html
+  # If nightly is breaking CI, modify this variable to target a specific nightly version.
   NIGHTLY_TOOLCHAIN: nightly
 
 # Sets the permissions to allow deploying to Github pages.

.github/workflows/validation-jobs.yml

@@ -13,6 +13,7 @@ concurrency:
 
 env:
   CARGO_TERM_COLOR: always
+  # If nightly is breaking CI, modify this variable to target a specific nightly version.
   NIGHTLY_TOOLCHAIN: nightly
 
 jobs: