Remove _all_ the trailing whitespace

Recently we tried to make the rustfmt bot remove trailing whitespace but
wrote the `sed` command incorrectly - it only removes a single
character.

Tested by running the new command on rust-bitcoin#4600.

Author: tcharding

.github/workflows/cron-weekly-rustfmt.yml

@@ -14,7 +14,7 @@ jobs:
           components: rustfmt
       - name: Run Nightly rustfmt
         # Run the formatter and manually remove trailing whitespace.
-        run: cargo +nightly fmt && find . -type f -name '*.rs' -exec sed -i 's/ $//' {} \;
+        run: cargo +nightly fmt && git ls-files -- '*.rs' -z | xargs sed -E -i'' -e 's/[[:space:]]+$//'
       - name: Get the current date
         run: echo "date=$(date +'%Y-%m-%d')" >> $GITHUB_ENV
       - name: Create Pull Request