github/workflows: only run build if source/build files change

When someone purely changes some documentation or something similar,
there's no point in having the whole CI build go off. This also leads to
the bot spamming a comment linking to artifacts. This is only useful if
actual source code changes (i.e. so users can test if need). So just add
a path filter to blacklist directories we don't want to consider. If all
changed files match those directories then the CI won't run.

.github/workflows/build.yml

@@ -6,8 +6,14 @@ on:
       - master
       - ci
       - 'release/**'
+    paths-ignore:
+      - 'DOCS/**'
+      - 'TOOLS/lua/**'
   pull_request:
     branches: [master]
+    paths-ignore:
+      - 'DOCS/**'
+      - 'TOOLS/lua/**'
 
 jobs:
   mingw: