Add script to update local snapshots from CI (#5816)

It seems like the thresholds are too low for all tests to pass when
snapshots are generated from windows / linux. We need a better solution
to this problem, but in the meantime this script should allow
contributors to update their snapshots by downloading them from the last
CI run.

scripts/update_snapshots_from_ci.sh

@@ -0,0 +1,36 @@
+#!/usr/bin/env bash
+# This script searches for the last CI run with your branch name, downloads the test_results artefact
+# and replaces your existing snapshots with the new ones.
+# Make sure you have the gh cli installed and authenticated before running this script.
+
+set -eu
+
+BRANCH=$(git rev-parse --abbrev-ref HEAD)
+
+RUN_ID=$(gh run list --branch "$BRANCH" --workflow "Rust" --json databaseId -q '.[0].databaseId')
+
+ECHO "Downloading test results from run $RUN_ID from branch $BRANCH"
+
+# remove any existing .new.png that might have been left behind
+find . -type d -path "*/tests/snapshots*" | while read dir; do
+    find "$dir" -type f -name "*.new.png" | while read file; do
+        rm "$file"
+    done
+done
+
+
+gh run download "$RUN_ID" --name "test-results" --dir tmp_artefacts
+
+# move the snapshots to the correct location, overwriting the existing ones
+rsync -a tmp_artefacts/ .
+
+rm -r tmp_artefacts
+
+# rename the .new.png files to .png
+find . -type d -path "*/tests/snapshots*" | while read dir; do
+    find "$dir" -type f -name "*.new.png" | while read file; do
+        mv -f "$file" "${file%.new.png}.png"
+    done
+done
+
+echo "Done!"