ci: ensure filenames don't have undesired characters (#2268)

This should ensure that files are valid across OSs - I've gone with an
inclusive pattern which purposely does not have some characters that are
technically legal but that I don't think there's a good reason to use
them in a filename.

Author: G-Rath

.github/workflows/checks.yml

@@ -42,6 +42,16 @@ jobs:
         with:
           persist-credentials: false
       - run: scripts/report_uncleaned_snapshots.py
+  filenames:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+      - run: |
+          find . -mindepth 1 ! -regex '.*/[#@A-Za-z0-9._-]*' -print0 \
+            | xargs -0 -I{} bash -c \
+              'printf "::error file=%q::This filename contains undesired characters\n" "$1" && false' _ {}
   format:
     permissions:
       contents: read # to fetch code (actions/checkout)