Use recent lock file when checking API files

Recently we added `--locked` to all the `cargo` invocations in the
script to check API text files. But if one has an updated lock file
(newer than recent) then this achieves nothing. I.e., if one has run
`cargo` _without_ `--locked` then the local lock file will have deps
that are too new (*cough* serde I'm looking at you). 

Copy the recent lock file into place when the script is run locally
but preserve the lock file state once the script is finished running.

Author: tcharding

contrib/check-for-api-changes.sh

@@ -24,9 +24,18 @@ main() {
     need_nightly
     need_cargo_public_api
 
+    # If script is running in CI the recent lock file is copied into place
+    # already by the github action job. Locally be kind to the environment.
+    if [ "${GITHUB_ACTIONS:-}" != "true" ]; then
+        [ -f "Cargo.lock" ] && mv Cargo.lock Cargo.lock.tmp
+        cp Cargo-recent.lock Cargo.lock
+    fi
+
     # Just check crates that are stabilising.
     generate_api_files "units"
 
+    [ -f "Cargo.lock.tmp" ] && mv Cargo.lock.tmp Cargo.lock
+
     check_for_changes
 }