Bump cargo-diet and run it slightly more cleanly

Changes:

- Bump the version of `cargo-diet` used in the `lint` job from
  1.2.4 to 1.2.7.

- Use the version tag not just as an operand to `--tag`, but also
  as the ref from which the installation script itself is obtained.
  (The rationale is that the effect of skew here would probably be
  unintuitive, and also that we had been specifying `master` but
  the default branch of the `cargo-diet` repository is now `main`.)

- Instead of using `|| true` on the step that runs
  `just check-size` (which needs cargo-diet), split installing
  and running into separate steps, and mark the running step
  `continue-on-error`.

Author: EliahKagan

.github/workflows/ci.yml

@@ -180,14 +180,17 @@ jobs:
         run: just doc
       - name: Run cargo fmt
         run: cargo fmt --all -- --check
-      - name: Run cargo diet
+      - name: Install cargo diet
+        env:
+          CARGO_DIET_TAG: v1.2.7
         run: |
-          curl -LSfs https://raw.githubusercontent.com/the-lean-crate/cargo-diet/master/ci/install.sh |
-            sh -s -- --git the-lean-crate/cargo-diet --target x86_64-unknown-linux-musl --tag v1.2.4
-
-          # Let's not fail CI for this, it will fail locally often enough, and a crate a little bigger
-          # than allows is no problem either if it comes to that.
-          just check-size || true
+          curl -LSfs "https://raw.githubusercontent.com/the-lean-crate/cargo-diet/refs/tags/$CARGO_DIET_TAG/ci/install.sh" |
+            sh -s -- --git the-lean-crate/cargo-diet --target x86_64-unknown-linux-musl --tag "$CARGO_DIET_TAG"
+      - name: Run cargo diet
+        run: just check-size
+        # Let's not fail CI for this, it will fail locally often enough, and a crate a little bigger
+        # than allows is no problem either if it comes to that.
+        continue-on-error: true
 
   cargo-deny:
     runs-on: ubuntu-latest