Fix documentation check building every target N times

The yq expression used to read documentation targets from .spi.yml wrapped
the config in select(.documentation_targets[] != ""). Because the filter
inside select() evaluates .documentation_targets[] as a stream of booleans,
select() re-emits the config once per target, and the trailing
.documentation_targets[] then multiplies the list to N*N entries.

For a package with 17 doc targets this expanded to ~272 targets, so
check-docs.sh built every target ~17 times and the job hit its 20 minute
timeout (appearing to hang / infinite loop).

Move the empty-string guard onto each individual target so every target is
emitted exactly once.

Author: rintaro

.github/workflows/scripts/check-docs.sh

@@ -94,7 +94,7 @@ if ! command -v yq &> /dev/null; then
 fi
 
 if [ -z "${docs_targets}" ] ; then
-  docs_targets=$(yq -r ".builder.configs[] | select(.documentation_targets[] != \"\") | .documentation_targets[]" .spi.yml)
+  docs_targets=$(yq -r ".builder.configs[].documentation_targets[] | select(. != \"\")" .spi.yml)
 fi
 
 package_files=$(find . -maxdepth 1 -name 'Package*.swift')