only check for bans if the dependency tree changed (#9252)

# Objective

- CI job `check-bans` fails often for unrelated reasons to a PR. Reduce
those failures

## Solution

- Currently, the job only runs if a `Cargo.toml` file changed. This PR
would run the job only if the output of `cargo tree --depth 3` is
different in a PR from main. the job would still always run on main

.github/workflows/dependencies.yml

@@ -29,11 +29,36 @@ jobs:
  check-bans:
  runs-on: ubuntu-latest
  steps:
+ # on main, prepare a new cargo tree output and cache it
+ - name: On main, prepare new cargo tree cache
+ if: github.ref == 'refs/heads/main'
+ run: cargo tree --depth 3 > cargo-tree-from-main
+ - name: On main, save the new cargo tree cache
+ if: github.ref == 'refs/heads/main'
+ uses: actions/cache/save@v3
+ with:
+ path: cargo-tree-from-main
+ key: cargo-tree-from-main 
+ # on other branch, restore the cached cargo tree output
+ - name: On PR, restore cargo tree cache
+ uses: actions/cache/restore@v3
+ if: github.ref != 'refs/heads/main'
+ with:
+ path: cargo-tree-from-main
+ key: cargo-tree-from-main
  - uses: actions/checkout@v3
  - uses: dtolnay/rust-toolchain@stable
+ # if not on main, check that the cargo tree output is unchanged
+ - name: Check if the cargo tree changed from main
+ if: github.ref != 'refs/heads/main'
+ continue-on-error: true
+ id: cargo-tree-changed
+ run: diff cargo-tree-from-main <(cargo tree --depth 3)
  - name: Install cargo-deny
  run: cargo install cargo-deny
+ # if the check was not a success (either skipped because on main or failed because of a change), run the check
  - name: Check for banned and duplicated dependencies
+ if: steps.cargo-tree-changed.outcome != 'success'
  run: cargo deny check bans
 
  check-licenses: