Ignore all build dirs in bin/check-source (#1291)

The `bin/check-source` script performs checks to ensure that headers are
correctly updated in the modified files in a commit. This script ignored
certain build directories since they contain generated Swift files. Some
build directories were missing in this list (particularly the build
directory emitted by `make-test-bundle`). The script has been modified
to indiscrimanately ignore all `.build` directories.

Author: snprajwal

bin/check-source

@@ -134,8 +134,7 @@ EOF
   (
     cd "$here/.."
     find . \
-      \( \! -path './.build/*' -a \
-      \! -path './bin/benchmark/.build/*' -a \
+      \( \! -path '*/.build/*' -a \
       \! -name '.' -a \
       \( "${matching_files[@]}" \) -a \
       \( \! \( "${exceptions[@]}" \) \) \) | while read line; do